CANASTOTA, N.Y.  An intensely watched Congressional race here that has become a battleground over the future of the Republican Party took another surprising turn on Sunday, when the Republican candidate  who ended her campaign a day earlier  announced that she was endorsing the Democrat.

The endorsement only intensified the intra-party fighting that has characterized the bitter contest, as Republicans denounced their former nominee, Assemblywoman Dede Scozzafava, calling her a turncoat who was a member of the party in name only.

“She basically put aside any pretensions and threw in with the Democrats,” said Dick Armey, the former Republican House leader, who was among an ever-growing group of conservative leaders opposed to Ms. Scozzafava’s place on the Republican ticket. Those leaders embraced Douglas L. Hoffman, the candidate on the Conservative Party line.

The White House helped engineer Ms. Scozzafava’s turnabout, sensing an opportunity to turn the rift that has emerged between moderate and conservative Republicans to its advantage. Members of New York’s Democratic Congressional delegation also reached out to Ms. Scozzafava over the weekend and encouraged her to back the Democrat, Bill Owens, a lawyer from Plattsburgh, N.Y.