By Emily Cadei, CQ-Roll Call

Three Republican lawmakers on Wednesday endorsed Doug Hoffman’s third-party campaign for the special House election in New York — a slap in the face to the party’s leadership and the current top brass at the National Republican Congressional Committee.

Endorsing Hoffman in the 23rd District contest were Sen. Jim DeMint of South Carolina, Reps. Dana Rohrabacher of California and Tom Cole of Oklahoma, who also served as NRCC chairman from 2006 to 2008.

The party’s preferred candidate is GOP nominee Dede Scozzafava. Hoffman’s name will appear on the upstate ballots as the Conservative Party nominee.

The nods from the three Republicans further cement the fact that Hoffman has moved from fringe candidate to a mainstream alternative on the right.

In his endorsement letter, Cole called Hoffman “the only Republican who can win this special election.” Democrats seem to be coming around to that viewpoint, too, shifting their fire from Scozzafava to Hoffman in the latest Democratic Congressional Campaign Committee ad, and seeking to fundraise off the possibility that he could be elected to Congress.

Cole, who is a member of the Republican Steering Committee, wrote that Hoffman “clearly represents the mainstream values and positions of the Republican Party.” He declined to directly criticize Scozzafava, the subject of countless attacks from the right for her moderate record in the state Assembly, but ticked off a number of issue areas where she and Hoffman diverge. “He opposes the stimulus, cap and trade, card check, the Democratic health care bill and the Obama administration’s reckless spending binge. He is a pro-life fiscal conservative who is committed to restraining the growth of government,” Cole wrote.

Rohrabacher called Hoffman “the only candidate in the race that shares my conservative Republican principles” and DeMint wrote in a piece on conservative blog RedState.com that “Doug Hoffman has stepped forward and offered voters a better choice” in the race to succeed Republican Rep. John M. McHugh , who resigned in September to become secretary of the Army.

The endorsements came on the same day the Hoffman campaign launched a new television ad featuring another GOP politician, former Sen. Fred Thompson of Tennessee (1994-2003), who deems the candidate a “principled conservative.” The anti-tax group Club for Growth, whose endorsement of Hoffman in September provided the campaign its initial surge of momentum, also announced it would begin airing a new TV ad Tuesday in Watertown, Burlington, and Syracuse highlighting the differences between Hoffman and attorney Bill Owens , the Democratic nominee. Scozzafava is not mentioned.

“This special election is now between Hoffman and Owens, and their economic records couldn’t be more different,” Club for Growth President Chris Chocola said in a release.

In an e-mail to DCCC supporters Tuesday, veteran Democratic strategist Paul Begala raised the specter of Hoffman’s candidacy in the upstate New York race to solicit contributions. “The inmates have taken over the asylum, and are abandoning the Republican candidate in favor of the extreme conservative,” Begala wrote.

“This could either be a cause for concern or a great opportunity,” he continued, urging readers to contribute to the DCCC and help get a “Democrat elected in a district that hasn’t sent a Democrat to Congress since 1852.”

CQ Politics currently rates the race a Tossup.

To see how the 2010 House races are shaping up, check out the CQ Politics election map