The American Civil Liberties Union offered to support the Rev. Jerry Falwell in his challenge of Virginia laws that restrict how much property a church can own.
Though Falwell often chides the activist group, the offer was welcomed by Jerry Falwell Jr., who is representing his father in the case.
Falwell Jr. said he hopes other groups also come forward, especially churches.
"It will make more of a statement if other denominations support us," the younger Falwell said.
The ACLU offered to file a friend-of-the-court brief in the federal suit against the state and the city of Lynchburg.
"We agree with your position that such laws discriminate against religion in both purpose and effect," Rebecca Glenberg, the ACLU's legal director, wrote in Tuesday's letter.
Kent Willis, executive director of the state ACLU, said his organization has always operated along clear legal lines, which sometimes makes for "very different bedmates. ... This is an instance we believe Rev. Falwell is absolutely right."
Falwell is a longtime critic of the ACLU. Two days after the Sept. 11 terrorist attacks, he said on Pat Robertson's "The 700 Club" show that God allowed the attack because of the work of abortion rights supporters, feminists and civil liberties groups, specifically the ACLU. Falwell later apologized.
Falwell's suit, filed Nov. 9, grew out of plans for a new sanctuary at his church, Thomas Road Baptist. Thomas Road could not own the sanctuary because state law prohibits any church from owning more than 15 acres in a city and 250 acres in a county. A municipality can increase its local limit to 50 acres, which Lynchburg did in the 1980s.
Jerry Falwell Jr. said Virginia and West Virginia are the only states that have such laws.
The suit said Virginia is violating the First Amendment right to free expression of religion and freedom of assembly, as well as the 14th Amendment's prohibition against excessive government entanglement with religion.
Falwell's suit also challenges laws that prohibit churches from becoming incorporated and require court oversight for land transactions.
"We're not filing this suit just for Thomas Road Baptist Church," Falwell said, "but for every church in Virginia."