Sen. Elizabeth Warren, D-Mass., should she launch a White House bid and win, would be the most pro-union president since Harry Truman. Warren has long history of strong pro-labor positions that put her well to the left of former President Barack Obama, himself a staunch union ally.

In her New Year’s Eve announcement of her exploratory committee, Warren cited boosting unions as a key reason why she was running. “Billionaires and big corporations decided they wanted more of the pie, and they enlisted politicians to cut them a fatter slice. They crippled unions so no one could stop them,” she said.

Building up unions’ political strength has been a goal of hers since taking office. “I love being here with labor. It reminds me of a family reunion. Lots of roughhousing. Occasional arguments. Plenty of food,” Warren said at the AFL-CIO's 2013 convention in Los Angeles, not long after first taking office. “But ultimately, in a family, we know we have got each other’s back.”

Warren has followed through by introducing legislation to benefit unions. She has also used her position on the Health, Education, Labor, and Pensions Committee to tear into Republican nominees to the Labor Department and other agencies.

The announcement of her presidential run was welcome news to her union allies. “I applaud @senwarren for calling out the attack on unions, racial injustice and the use of fear and hatred to distract and divide us,” tweeted Mary Kay Henry, president of the Service Employees International Union. “I agree that we need an economy and democracy that works for all of us. We must also fight for #UnionsForAll no matter where you work.”

An official at a major national union, who requested anonymity because the union isn’t ready endorse a candidate, said there is genuine grass-roots enthusiasm for Warren. There is a tendency among candidates, the official said, to play up union issues as a way to appeal to the Democratic base, even when they cannot follow through on promises. Warren, however, has the reputation of somebody who doesn’t just talk the talk.

“A lot of rank-and-file labor activists are quite excited for her because of the profile she has built for herself in her short time in Congress,” the official said. “She has absolutely championed good legislation since she got into office.”

At the same time, there are other pro-labor liberals among the potential Democratic candidates, the official noted, citing Sens. Cory Booker of New Jersey and Kamala Harris of California. Warren may have to fight other nominees for labor’s backing, which would force her to make more promises to them. “There are other candidates with voting records as good as hers,” the official noted.

The AFL-CIO's legislative scorecard says that, over her career, Warren has voted with unions 98 percent of the time. The only occasion when Warren opposed the union federation’s lobbying in her entire time in the Senate was on a 2015 transportation spending bill that the unions backed. Warren objected to a Republican provision in the bill that she said would weaken the Consumer Financial Protection Bureau, a federal agency she was involved in creating during the Obama administration.

Warren’s boldest attempt to help unions was introducing the Protecting Workers and Improving Labor Standards Act, a 2017 bill that would have repealed all state right-to-work laws. She also co-sponsored the Workers’ Freedom to Negotiate Act of 2018, legislation written by Sen. Patty Murray, D-Wash., that included similar repeal language. Neither bill made it out of committee.

Right-to-work laws prohibit private sector workers from being compelled to join or financially support a labor union as a condition of employment. Currently 27 states have such laws, including five that have enacted them in the last seven years. The laws don't prevent workers from joining or forming unions, but rather give dissenters the ability to opt out.

From Warren’s perspective, right-to-work laws weaken unions and cause them to have difficulty retaining members and funding. “If we want to protect workers and expect a level playing field in international trade deals, we need to start at home — and that means banning states from imposing restrictions that prevent workers from joining together to fight for their future,” she said in September 2017.

The legislation would have been the biggest change to labor laws in about seven decades, said Mark Mix, president of the National Right To Work Legal Defense Foundation. Right-to-work hasn’t even been seriously debated in the Senate since the mid-1960s, he noted.

“Her ideology really comes from socialism: We know what is good for you and therefore shut up and listen to us,” Mix said. “The whole litany of bills she has introduced is to give new, expansive powers to organized labor.”

[Also read: Unions criticize USMCA wages provision, potentially endangering the deal]

Labor support is important to Democrats as a major source of campaign funding. Unions gave more than $59 million in the 2016 election cycle alone, according to the Center for Responsive Politics. The conservative Center for Union Facts put the amount that unions gave to Democrats, liberal activist groups, and political action committees between 2012 and 2016 at $765 million.

Warren also co-sponsored the 2017 WAGE Act, a union wish list of changes to federal laws that would have made workplace-organizing elections much easier to win. It would have allowed union “card check” elections, scrapping federally monitored secret-ballot votes in favor of allowing unions to submit cards supposedly signed by workers instead. The legislation would also have sped up elections, limiting management’s ability to contest them, and drastically raised the penalties for election interference by management.

As a member of the HELP committee, Warren was a major critic of President Trump’s first nominee for labor secretary, fast food mogul and outspoken conservative Andy Puzder. Democrats repeatedly delayed his confirmation vote, raising concerns over his personal investments and his actions running franchises such as Carl’s Jr. Warren was the most aggressive, at one point sending Puzder a 28-page list of questions. He eventually withdrew his nomination when his Senate Republican support collapsed.

The senator has also been an ardent critic of Trump’s nominees to the National Labor Relations Board, the main federal labor law enforcement agency. It was inquiries from her office regarding whether NLRB members should have recused themselves from the 2017 Hy-Brand case that ultimately resulted in the board vacating the decision last year. Hy-Brand was watched closely by the business community because it involved situations in which a “joint employer” with another business was legally responsible for workplace violations. The board’s vacating the decision meant that a much broader liability standard instituted by the Obama administration remained in place.

Warren has since demanded that two of the NLRB’s members, Chairman John Ring and Board Member William Emanuel, both Trump appointees, recuse themselves from a broad swath of cases. Doing so would effectively return the board to Democratic control.

A spokesman for the senator did not respond to request for comment.