Arlen Specter, D-Pa.

The prime example of this is in Kentucky, a conservative state that never much cared for Barack Obama in 2008 and that has turned even more sharply against him, and against the national Democratic Party, since his presidency began. This, coupled with the feeble economy and the basic buyer’s remorse nature of midterm elections, should make the contest to replace retiring Sen. Jim Bunning a cakewalk for the GOP.

But it isn’t, because the GOP base, in a revolt against a party establishment that it believes has betrayed conservative principles, opted for Rand Paul in the May 18 primary, instead of Trey Grayson, the establishment’s choice. Within 24 hours, Paul was scrambling to prove to a national audience that he doesn’t actually oppose the Civil Rights Act of 1964. He’s now clammed up and shaken up his campaign staff, but Paul is an unseasoned candidate and a true believer. With more than five months to go until the election, further embarrassments seem inevitable.

Democrats, in other words, could end up stealing a Republican-held seat in a conservative state. The more controversial Paul becomes — the more he becomes the main issue in the Kentucky race, instead of Obama, Washington Democrats and the economy — the more conceivable it becomes that swing voters who are otherwise ready to vote against the Democrats will end up voting against Paul instead.

This, obviously, wouldn’t have been the case if Grayson were the GOP nominee. In a year like 2010, with the national climate so favorable to their party, there’s virtue for Republican candidates in being inconspicuous — something that the bland, cookie-cutter Grayson would have had no trouble pulling off. Paul? Not so much.

This same basic dynamic — the GOP base forcing the nomination of a deeply flawed candidate — could end up playing out in as many as nine Senate battleground venues, potentially saving Democrats from an electoral drubbing.

Take Nevada, where Majority Leader Harry Reid should, by most historical measures, be a goner. His polling numbers are poisonous, his long-term incumbency and D.C. insider status are severe drags, and his state’s independent voters seem eager to use the midterm to “send a message” to Obama.

But it’s looking more and more like the GOP will nominate Sharron Angle, a far-right former assemblywoman who has the ardent backing of the Tea Party movement. Like Paul in Kentucky, Angle herself could easily become an issue in the fall campaign. Making the race a referendum on the GOP candidate is Reid’s only hope. If Angle’s his foe, he’ll have a chance to do that.

Or there’s Florida, where the GOP base’s revolt against Charlie Crist has flushed the popular governor out of the party — and into an independent Senate bid that is attracting significant Democratic support. If Crist prevails, there’s now reason to believe he’ll vote with Democrats to organize the Senate next year. And in California, the flaky Carly Fiorina has marginalized another Tea Party favorite and surged ahead of moderate Tom Campbell in the GOP primary. If she wins the nomination, Fiorina will face Barbara Boxer, whose popularity is now ominously low — and who is surely grinning at the prospect of dodging Campbell.

The GOP base could also topple establishment favorites in primaries in Colorado, New Hampshire, Washington and Arizona — ideological triumphs that would severely complicate GOP prospects in fall races that would otherwise be very winnable. And the party base’s revolt in Pennsylvania last year, which prompted Arlen Specter to switch parties, has left the GOP with Pat Toomey, who faces an iffy fall campaign against Joe Sestak, who knocked off Specter in the primary.

Right now, Republicans need to pick up nine seats to draw even with the Democrats in the Senate (at which point Republicans could potentially win Joe Lieberman over) and ten to gain control of the chamber outright. The good news for them is that — especially with this week’s decision of Dino Rossi, an electable establishment favorite, to run for Patty Murray’s seat in Washington — there are (potentially) enough vulnerable Democratic incumbents to hit those numbers. And the national political climate only helps. The bad news is that just every time a Tea Party favorite wins a primary, it means the GOP won’t be fielding his strongest fall candidate — which could help Democrats claim seats they have no business winning in this environment.

This may be the biggest single difference between 2010 and the GOP revolution of 1994 — which many Republicans believe they can replicate this fall. In ’94, the party base was about as cranky and agitated as it now is; this is just how the right gets when it’s denied White House and congressional control. But that anger was largely trained on Bill Clinton and the Democratic Congress. The GOP base generally trusted — or at least was willing to work with — its party’s Washington establishment.

Not so in 2010. Now, the GOP base distrusts the establishment as much as it distrusts the Democrats. It’s a consequence of 1994. As Salon’s Gabriel Winant has noted, the election of a GOP Congress in ’94 (and then a GOP president in 2000) fundamentally altered the base’s expectations: It was time for the establishment to take all of its lip service to the base and put it into law.

But that, of course, didn’t happen. Newt Gingrich backed off his confrontation with Bill Clinton in 1995, George W. Bush campaigned on “compassionate conservatism,” and a Republican president and Congress spent the aughts expanding government and racking up massive debt.

No wonder that today, unlike 1994, the Republican base is unwilling to take the establishment at its word. The consequence is the proliferation of Tea party candidates — a proliferation that might just save the Democratic Congress come November.

Sure, it could simply be that Sestak, who was barely known to Pennsylvanians even on primary day, is enjoying the kind of bounce that often accompanies generally positive saturation news coverage. But it’s also easy to see that in opting for him over Specter, Democrats made a smart fall bet.

The very attributes that were supposed to make Sestak an easy mark — his lack of name recognition, experience and polish — are all working for him right now. He’s been in Congress since 2007, but he feels new to most Pennsylvanians. In the last week, he’s been introduced to them as a gutsy outsider who took on the Democratic establishment and knocked off a calculating, five-term incumbent. This image is a perfect match for the moment, with marginal voters frustrated with the ruling Democrats (part of the buyer’s remorse nature of most midterm elections) and furious with the Washington establishment in general. It’s almost impossible to conceive of Specter keeping these voters in the Democratic fold in November. But Sestak at least has the potential to.

Democrats in Arkansas may be on the verge of a similar switch-out, with Blanche Lincoln potentially facing defeat in a June 8 Democratic runoff. As with Specter, national Democrats have touted Lincoln as their best — and maybe only — hope of holding on to the seat. By conventional standards, this would probably be true: She’s succeeded in conservative Arkansas in part by defining herself in opposition to the national Democratic Party on key issues. But in 2010, the evidence suggests, Arkansans don’t see her as a “centrist.” They simply see her as a Washington insider — and a member of the party they’re already inclined to vote against.

Her Democratic foe, meanwhile, would ordinarily be seen as too liberal to win in the fall. Lt. Gov. Bill Halter has received substantial support from organized labor and from national liberal groups that are angry with Lincoln. Halter has avoided using the “l-word,” but he’s been running against Lincoln from the left. And yet, last Tuesday’s preliminary election returns — which showed Halter besting Lincoln in conservative pockets of the state — suggest Arkansans don’t see him as a lefty; they see him as an outsider. And that reputation will only grow if Halter manages to knock off Lincoln on June 8. Winning in the fall would still be tough, but Halter — like Sestak — clearly offers his party its best chance.

This same game can also work at the state level. Take New York, where state Attorney General Andrew Cuomo formally announced his gubernatorial candidacy over the weekend. Cuomo is only running because the incumbent, David Paterson, has been rendered politically toxic, in part because of his own blundering and in part because of voters’ general rage toward Albany. Cuomo, despite his pedigree, has managed to craft an outsider’s image, using the A.G.’s office to score enviable headlines while avoiding the taint that comes with dealing with the Legislature (or grappling with the state budget). Paterson would lose to just about any Republican this fall, but Cuomo — in polling released on Monday — leads each of the three prospective GOP candidates by more than 40 points.

Or there’s Colorado, where first-term Democratic Gov. Bill Ritter declined earlier this year to seek reelection. Like many other governors, he’d seen his numbers drop as the economy and his state’s budget woes worsened. A poll last week gave Ritter an approval rating of just 34 percent. And yet that same poll found the Democrats’ replacement candidate, Denver Mayor John Hickenlooper, notching a favorable score of 47 to 33 percent. In a general election trial heat, Hickenlooper and Republican Scott McInnis are tied at 44 percent — a far cry, no doubt, from where Ritter would be if he’d stuck it out.

In most races this fall, Democrats will be handicapped by their party affiliation. But by replacing embattled incumbents with “outsiders,” they can level the playing field in some crucial ways.

That said, this approach will probably be of limited use to the party this year. For one thing, the Democratic establishment (just like the GOP establishment) instinctively rallies around its incumbents. So efforts from the party’s leadership to nudge aside vulnerable incumbents have been (and will continue to be) limited. Democrats were able to gently persuade Chris Dodd to give up in Connecticut back in January, but that’s been the exception, not the rule.

Then there’s the calendar. Sestak’s electability advantage (and potentially Halter’s) has come as a genuine eye-opener to many Democrats. Only now are they recognizing how poisonous incumbency really is this year. You can argue that they should have seen it coming — it’s pretty clear, for instance, that Democrats would have had a better chance in last year’s New Jersey gubernatorial race if they’d replaced the doomed Jon Corzine with former acting Gov. Richard J. Codey — but as a practical matter, it’s now too late to organize credible primary challenges in most states and districts.

Still, if Democrats end up holding on to the Pennsylvania or Arkansas seats (or both), it almost certainly would be enough to save their Senate majority — rather impressive, considering the party establishment’s warning that throwing out incumbents would jeopardize both seats.