“It’s a tough vote, but leadership’s got to do what leadership’s got to do,” said Rep. Kurt Schrader (D-Ore.), one of seven Democrats who voted no. “Labor’s very insistent.”

The logjam broke last month after 72 Democrats wrote House Speaker Nancy Pelosi to demand that the bill be brought to the floor. AFL-CIO President Richard Trumka told Democrats Wednesday that he will withhold campaign cash from any Democrat who opposes the bill.

“Do not ask the labor movement for a dollar or a door knock,” Trumka said. “We won’t be coming.”

Supporters of the bill say it would put workers on more equal footing with employers, raising wages and improving working conditions nationwide. Democrats also hope to drive a wedge between President Donald Trump, who opposes the bill, and white working class voters after the president's recent legislative victory passing the U.S. Mexico-Canada agreement. That agreement is expected to create jobs in the Midwest-based auto industry.

But businesses and many Republicans regard the bill as a declaration of war. In the weeks leading up to the vote, business groups sought to peel off vulnerable Democrats and to stop additional Republicans from signing on.

The White House said this week that the legislation would “take the country in precisely the opposite direction from the president’s successful deregulatory agenda, which has produced rising blue-collar wages and record low unemployment.“

Among its many proposed changes, the PRO Act would allow employees to form unions through "card check" (that is, through the informal collection of authorization forms from a majority within the bargaining unit), when an employer has demonstrated interference in an election. The bill would also grant the National Labor Relations Board power to levy punitive fines on employers that violate labor law. Right now, the NLRB can collect only back pay.

The bill would also make it harder for employers to classify workers as independent contractors under the National Labor Relations Act, making more workers eligible to join unions. The bill would also make it nearly impossible for states to enforce right-to-work laws.

Five Republicans voted for the bill, including co-sponsors Jeff Van Drew and Chris Smith of New Jersey and Brian Fitzpatrick of Pennsylvania.

Unions have sought, unsuccessfully, to pass one variation or other of PRO Act since 1947, when a Republican Congress passed (over President Harry Truman's veto) the Taft-Hartley amendments to the 1935 National Labor Relations Act, a seminal labor law that is often described as "labor's Magna Carta." Taft-Hartley rolled back union protections in response to a wave of postwar strikes and in anticipation of a postwar economic depression that never came. Taft-Hartley opened the door for more than two dozen states to pass "right-to-work" laws that prohibit unions from compelling union nonmembers to pay fees to cover their share of the costs of collective bargaining.

One steep obstacle to strengthening labor rights in the postwar era was a decline in public approval for labor unions that began in the early 1960s as congressional investigators and the Justice Department exposed illegal union activities, most notably Teamsters President Jimmy Hoffa's ties to the Mafia. Labor's Gallup approval rating fell from 75 percent in the late 1950s to 48 percent during the Great Recession of 2007-9.

With the start of the economic recovery, however, public opinion of labor unions climbed back up, and today organized labor's Gallup approval rating stands at 64 percent.

