The president of the International Brotherhood of Teamsters praised one Republican congressman on national television. A major transportation union rated another among its most loyal supporters in Congress. And an influential building trades group said a third would help begin “restoring the sense of unity” the country had lost.

After years of antagonism between the labor movement and Republican Party leadership, a growing number of moderate Republican lawmakers are trying to break from their party’s union-hostile policies as they defend House swing seats this fall, and they have found surprising support.

The shift remains nascent, largely limited to building trades and other blue-collar groups in the union-heavy Northeast. Yet, with the elections looming, it has already injected an unpredictable new element into a half-dozen races that could determine control of the House of Representatives.

In New York’s Hudson Valley, Representative Mike Lawler has collected tens of thousands of dollars more in union donations than his Democratic opponent. Further upstate, a 1,500-member electrical workers’ union that once opposed Representative Marc Molinaro is now working to re-elect him. And in New Jersey, the state’s Building and Construction Trades Council not only flipped sides to support Representative Tom Kean Jr. but helped keep the Democrat-aligned A.F.L.-C.I.O. on the sidelines.

“Institutionally, the Republicans are still anti-worker,” said John Samuelsen, the head of the 150,000-member Transport Workers Union of America, a group that has turned heads by wading into several hard-fought contests to back Republicans. But he said he was also frustrated with the Democrats who appeared to be taking union support for granted.