The 2020 Democratic presidential candidates are falling all over themselves to win support from union leaders and members on the road to the White House. “I am a union man, period,”says Joe Biden. “I stand with the @Palms workers who are fighting for strong wages, comprehensive benefits, and dignity on the job,”tweets Elizabeth Warren. Bernie Sanders proposes a plan to double union membership. Kamala Harris wants teachers to get a $13,500 raise.

That’s a lot of labor love, considering that Democrats in general have done little to stanch union losses in the past few decades. The percentage of workers in unions is at its lowest in more than a century – down to 10.5 percent from a peak of 35 percent in 1954. Still, labor remains a traditional Democratic constituency, even though Hillary Clinton won union voters by just eight percentage points in 2016, the narrowest margin for a Democrat since 1984. Democrats want that vote back, and they particularly need it in three states that flipped from blue to red in 2016.

Picking up union and blue-collar support in Pennsylvania, Michigan and Wisconsin could be critical to winning the presidency – but it will be a tough haul. Union membership in those states has fallen steeply over the past decade, and Republican lawmakers have done their utmost to hasten the decline.

Take Wisconsin. Gov. Scott Walker’s 2011 legislation to gut public-employee unions there has had a far greater effect on labor rolls than many people realize. Largely because of that anti-union law, union membership in Wisconsin has fallen at a faster rate over the past decade than in any other state,plunging by 44 percent, or 177,000 workers, since 2008. Job losses from the Great Recession also fueled the decline, and so did an anti-union-fee directive, often called a right-to-work law, passed by Walker’s Republican allies in the legislature. Such provisions let employees at unionized workplaces decline to pay any dues or fees to the unions that bargain for them. This encourages some workers to quit the union and stop paying dues.

Walker insisted at the time that he wanted to hobble government employee unions for just one reason: to hold down costs on wages, health coverage and pensions. But one of Walker’s top allies, Scott Fitzgerald, the Senate majority leader, let slip a partisan motive behind the law. “If we win this battle, and the money is not there under the auspices of the unions,” Fitzgerald said, “. . . President Obama is going to have a much more difficult time getting elected [in 2012] and winning the state of Wisconsin.” Obama won Wisconsin in 2012, but Donald Trump narrowly won it in 2016.

There are 177,000 fewer Wisconsin workers in unions, but Trump won the state by just 22,748 votes. Had those workers not left unions – and if their dues had helped fund a more robust pro-Clinton, anti-Trump message to them and others – might Clinton have won there?

The story is similar in Michigan. Between 2008 and 2018, union membership there dropped by 19 percent, or 146,000 workers, to 625,000. Trump won Michigan by just 10,704 votes. The decline in membership is largely attributable to the auto industry’s hard times during the recession – and to a right-to-work law that the Republican-dominated legislature enacted in 2013. One study found that as a result of that law, Michigan’s largest unions reduced political spending by $26 million, or 57 percent.

Henry Farber, a labor economist at Princeton, found that right-to-work laws, by allowing non-dues-paying “free riders,” substantially shrink union treasuries. And David Ellwood, former dean of the Kennedy School of Government at Harvard, and Glenn Fine, a former Justice Department official,found in another study that right-to-work laws, absent other factors, caused a 5 to 10 percent drop in union membership.

Roland Zullo, director of the Center for Labor and Community Studies at the University of Michigan at Dearborn, found that the share of blue-collar workers saying they had been contacted in get-out-the-vote drives fell by 11 percent after right-to-work laws were enacted. (There was no comparable effect on white-collar workers.)

In a 2018 study, James Feigenbaum of Boston University, Alexander Hertel-Fernandez of Columbia University and Vanessa Williamson of the Brookings Institution concluded that anti-union right-to-work laws caused the Democratic vote share to drop by 3.5 percent in states that passed such laws. That was far more than Trump’s winning margin in Michigan (0.2 percent) and Wisconsin (0.8 percent).

The story is somewhat different in Pennsylvania because Republicans there have not been able to push through an anti-union-fee law. Still, stung by the Great Recession, Pennsylvania’s unions have lost 182,000 members since 2008, falling by 21 percent to 665,000; Trump won Pennsylvania by 44,292 votes.

Many analysts attribute Clinton’s loss in those three supposed blue-wall states to her failure to connect well with blue-collar voters and to Trump’s wowing them with his impossible promises to bring all the jobs back and stop the flood of factory closings. But it certainly didn’t help that, year in and year out, Republican lawmakers and their billionaire-donor allies have pushed in myriad ways to further hobble unions – through legislation and through court cases like Janus v. AFSCME Council 31, in which the Supreme Court ruled last year that teachers, police officers and other government employees can’t be required to pay any dues or fees to the unions that bargain for them.

Grover Norquist, the anti-tax crusader, said that as a result of Walker’s anti-union law, known as Act 10, 135,000 government workers in Wisconsin stopped paying union dues. He said this had cut union revenue by an estimated $135 million a year. Noting that other Republican governors hoped to enact similar legislation – Iowa, for instance, passed Wisconsin-like legislation in 2017 – Norquist predicted: “If Act 10 is enacted in a dozen more states, the modern Democratic Party will cease to be a competitive power in American politics. It’s that big a deal.”

New York Times columnist Thomas Edsall noted in 2014 how concerned Republicans were about unions and how unconcerned many Democrats were. “A paradox of American politics,” he wrote, “is that Republicans take organized labor more seriously than Democrats do. The right sees unions as a mainstay of the left, a crucial source of cash, campaign manpower and votes. . . . If Republicans and conservatives place a top priority on eviscerating labor unions, what is the Democratic Party doing to protect this core constituency? Not much.”

That’s changing in this election cycle. I have written about labor unions and political campaigns for more than two decades, and never have I seen so much talk among Democrats about the importance of unions and helping unions expand as I’ve seen among this year’s presidential candidates. They saw how their party’s hopes were dashed in 2016 because Clinton lost Wisconsin, Michigan and Pennsylvania, key states that were longtime labor strongholds. Just as Republicans have seen that pulling down labor has been important to their political successes, Democrats are finally recognizing that building up unions and union membership could be vital to their successes in the 2020 presidential race and beyond.

The question is whether Republican legislatures have fatally stacked that deck.

Steve Greenhouse was the labor and workplace reporter for The New York Times for 19 years. He is the author of “Beaten Down, Worked Up: The Past, Present, and Future of American Labor.”