Barring some seismic shock to American politics, the next Congress will be a barrier to progressive change. And as much as the New Deal remains a touchstone for the left wing of the Democratic Party, it is also an important to keep in mind that the composition of Congress had as much to do with that particular heyday of American liberalism as the character of presidential leadership did.

The 1935 National Labor Relations Act, to give just one example, was a signature accomplishment of the New Deal. Devised in response to profound industrial unrest, it recognized the right to strike, to organize, to form a union. It gave American workers unheard-of power to shape the nation’s political economy, bringing the United States as close as it ever came to social democracy.

President Franklin Roosevelt is rightfully credited for signing the bill into law. But while his White House wanted major labor reform, this wasn’t his bill — it came to the White House, not the reverse. The architect was Senator Robert Wagner of New York, a longtime ally to labor and a staunch New Dealer. After the 1934 midterm elections gave Democrats a commanding majority in Congress — nearly obliterating the right wing of the Republican Party — Wagner saw a chance to act.

Working closely with organized labor, Wagner and his staff went for broke. They affirmed the right to collective bargaining, established an independent board (still operating as the National Labor Relations Board) to adjudicate labor disputes and created new protections for workers who want to unionize, as well as restrictions on the ability of employers to coerce their employees out of joining a union.

Wagner introduced the bill in February, 1935. It passed the Senate in May by a vote of 63 to 12. The House followed suit, and by the end of June it was on the president’s desk. And while Roosevelt quickly signed the bill, he had actually been ambivalent during the process, wary of antagonizing business interests ahead of his re-election campaign in 1936. (The F.D.R. who welcomed the hatred of reckless capitalists would come later.) Energetic, liberal lawmakers had forced his hand.