Barbara Boxer, D-Calif.

After two terms as governor and nine as the state’s lone congressman, Castle appears better positioned than other veterans who faced a tea party-backed challenge this year. If he prevails over Christine O’Donnell on Sept. 14 — he and GOP officials have launched a fierce counterattack — he would join more than a half-dozen other veteran Republican officeholders on the ballot in Senate races.

In matters of style as well as policy and political experience, they are the polar opposite of Rand Paul of Kentucky, Sharron Angle of Nevada and Ken Buck in Colorado. Those three tapped into an anti-government sentiment, espouse politically risky positions, won primaries over establishment candidates, and now face difficult races in the fall.

No matter the blend of candidates that Republicans end up with, a persistently weak economy and voter anger add up to enough competitive races to give them at least an outside chance of winning Senate control. Already, a constellation of outside groups is spending heavily on television in Senate races, including more than $5 million this summer for two groups backed by former George W. Bush political adviser Karl Rove.

Republicans need to capture 10 seats to win a majority, and as many as a dozen held by Democrats appear competitive, as well as at least five currently in the hands of the GOP.

Ironically, as the primary season draws to a close and the fall campaign dawns, both parties try to straddle politically inconvenient facts that underscore broader trends.

Democrats are loath to concede their majority is at risk. “I don’t think there are” enough competitive races for that to happen, said Sen. Robert Menendez of New Jersey, head of the party’s Senate campaign committee. Yet the party’s strategists issue a stream of statements saying that many tea party-backed challengers in tight contests are “too extreme” and will cost the GOP its chance of gaining control.

The committee is making a quick check to see whether it has a late, low-budget opportunity in strongly Republican Alaska, where tea party-backed challenger Joe Miller defeated GOP Sen. Lisa Murkowski in a recent primary.

Senate Republican leader Mitch McConnell of Kentucky and other lawmakers in his party tried repeatedly to defeat tea party-supported challengers in Kentucky, Colorado, Florida, Nevada and elsewhere in recent months, privately expressing fears they would prove unelectable.

Now, after compiling a mixed record in the primaries, the campaign chairman, Sen. John Cornyn of Texas, says the fall is “an opportunity for Republicans, independents libertarians and disgruntled Democrats to come together around a fiscal responsibility message and one that says the government can’t grow itself out of this problem.”

There was never any doubt that GOP strategists wanted Castle on the ballot. Arguably the most moderate Republican in the House, he also was viewed as the only contender with a chance to win the seat at a time when Beau Biden, the state attorney general and son of the vice president, seemed likely to run.

When the younger Biden opted not to run, enter Chris Coons, a lawyer now in his second term as executive of the largest of the state’s three counties.

Other veteran Republicans on Senate ballots this fall include Rep. John Boozman of Arkansas, whom party officials say needed some coaxing to run. Now he leads Democratic Sen. Blanche Lincoln by significant margins in public and private polls.

Already, Democrats have tacitly written off a seat in North Dakota, where former GOP Gov. John Hoeven, initially a reluctant candidate, is favored to succeed retiring Democratic Sen. Byron Dorgan.

Former GOP Sen. Dan Coats is ahead in the polls as he tries to win back an Indiana seat he voluntarily gave up a dozen years ago.

Next door in Illinois, Republican Rep. Mark Kirk is in a tougher race with Democrat Alexi Giannoulias for the seat President Barack Obama once held. Kirk, too, was courted heavily by Cornyn and others.

Republican veterans also are on the ballot in important Midwestern races where GOP senators are retiring. In Ohio, former Rep. Rob Portman, who served in two Cabinet-level positions in the Bush administration, polls ahead of Democratic Lt. Gov. Lee Fisher and had a multimillion-dollar cash-on-hand advantage in the most recent fundraising report.

Missouri Rep. Roy Blunt, a former member of the House Republican leadership, is in a competitive contest with Democratic Secretary of State Robin Carnahan for an open seat in GOP hands.

In other races that are tight heading into the fall campaign, the political pedigree of the Republican is mixed.

In Florida, former House Speaker Marco Rubio is a rarity, a tea party favorite who is also an accomplished politician. The three-way race with Democratic Rep. Kendrick Meek and Gov. Charlie Crist, a former Republican running as an independent, is one of the most unpredictable in the country.

In Pennsylvania, Pat Toomey, a former congressman and ex-head of the conservative Club for Growth, is running against Democratic Rep. Joe Sestak. Sestak defeated Sen. Arlen Specter in the Democratic primary.

Republicans got the recruit they wanted in Washington state this summer, and Dino Rossi is challenging Democratic Sen. Patty Murray.

In Wisconsin, California and Connecticut, where veteran Democrats are on the ballot, it’s the size of a candidate’s checkbook as much as ideology that mattered keenly to Republican recruiters.

Millionaire Ron Johnson, a political novice, is challenging Democratic Sen. Russ Feingold of Wisconsin. In Connecticut, where Democratic Sen. Chris Dodd is retiring, Linda McMahon, former CEO of World Wrestling Entertainment, spent millions of her own money to win the primary and has pledged to spend millions more against Democratic Attorney General Dick Blumenthal.

In California, former Hewlett-Packard CEO Carly Fiorina is challenging three-term Democratic Sen. Barbara Boxer in the costliest campaign state in the country.

In a difficult environment, Democrats also cite opportunities to pick up a seat.

High on the list is Missouri, followed by Kentucky, where Attorney General Jack Conway is running against Paul, and the complicated three-way Florida election. In Louisiana, Democratic Rep. Charlie Melancon is challenging Republican Sen. David Vitter.

Their claims have been more muted about the Delaware race, although the vice president is expected to make at least two more appearances in the state this fall.

Coons says he successfully restored his county to financial health and is ready to do the same for the federal government.

Treading carefully, at least for now, he says Castle is a “decent and likable man” but one who votes more and more like a conservative Republican while Delaware grows increasingly Democratic.

“He has lost or forgotten the courage to stand up to the increasingly conservative bent of his party,” Coons says, pointing to the congressman’s votes against the Obama administration’s economic stimulus legislation of 2009 as well as the landmark health care bill.

On the other hand, as an outsider, Coons complains that Castle voted for the financial bailout of 2008, adding it lacked accountability.

Castle’s rebuttal skips past any political implications of his votes in Congress.

The stimulus did little beyond creating temporary construction jobs, he says in an interview, and the administration lowballed the cost estimates for the health care bill. “Most of the banking (bailout) money has been repaid with interest.”

A Republican in a Democratic state, and a longtime moderate in a conservative party under pressure from the tea party movement, Castle talks of government and civility, not politics.

“Once we are elected I think we have a responsibility to sit down and work out our differences,” he said.

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Online:

Democratic Senatorial Campaign Committee: http://www.dscc.org/

National Republican Senatorial Committee: http://www.nrsc.org/