Labor leaders are “playing hard to get” with Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE in her bid for the Democratic presidential nomination.

Many of the nation’s top unions are sitting on the sidelines, content to let Clinton sweat it out while they withhold endorsements.

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Some labor officials are frustrated with Clinton for not coming to their aid in the fight over trade legislation in Congress, while others are skeptical of her commitment to their issues.

The face of the labor movement, AFL-CIO President Richard Trumka, has not endorsed Clinton while seemingly courting her biggest rivals in the Democratic primaries: Sen. Bernie Sanders Bernie SandersJacobin editor: Primarying Schumer would force him to fight Trump's SCOTUS nominee Trump campaign plays up Biden's skills ahead of Cleveland debate: 'He's actually quite good' Young voters backing Biden by 2:1 margin: poll MORE (I-Vt.) and Vice President Biden, who is weighing a run for president.

“Say you’re in love with a girl and want to marry her. She’s playing it cool. So you figure the best way to make her jealous is to flirt with someone else,” said Democratic strategist Brad Bannon.

“Trumka wants to marry Hillary, but until she’s willing to make stronger commitments to labor he’s going to flirt with Bernie and Biden,” Bannon added.

“That will get Hillary’s attention.”

The AFL-CIO noted that it is not uncommon for the labor organization to abstain from endorsing Democratic presidential candidates in the primaries and wait until the general election to get involved.

Clinton isn’t without labor support, having already secured endorsements from two powerhouse unions: the American Federation of Teachers and the International Association of Machinists and Aerospace Workers.

But other labor organizations such as the AFL-CIO, International Brotherhood of Teamsters, and Service Employees International Union (SEIU) have yet to back a candidate.

"They’re holding back, playing hard to get,” said Jim Manley, a Democratic strategist.

Sanders has picked up some union support as well, having been endorsed by the National Nurses United.

Clinton indicated Friday she expects to pick up labor endorsements moving forward.

"Labor Day is kind of a pivot for all kinds of political actions during a campaign year," she said. "I think there will be a number of other unions that will be endorsing, but they have to make those decisions. Whatever timetable they pursue."

She lauded her own history as a “strong advocate for organized labor” and added, "I'm not taking anything for granted. I'm going to work hard to win as much support as I possibly can."

Labor unions have for years been a core bloc of the Democratic coalition, spending millions of dollars each election cycle for advertising and grassroots organizing.

But labor officials feel burned by the Obama administration’s handling of trade, and are upset Clinton did not take a clear stance against the trade promotion authority bill (TPA) that passed Congress this summer.

TPA, also called fast-track, would allow Obama to send the Trans-Pacific Partnership (TPP) — a sweeping trade pact with Pacific Rim nations — to Congress for an up-or-down vote.

Unions fear the TPP will hurt manufacturers and send jobs overseas, and fought for months to keep the fast-track bill from passing.

Clinton repeatedly sidestepped her position on fast-track, mostly staying silent until the debate in Congress was nearly over.

“I certainly would not vote for it unless I was absolutely confident that we would get trade adjustment assistance," she said, referring to a program that was reauthorized alongside TPA.

Clinton could have changed the debate over the TPA bill by coming out forcefully against the legislation. Because she didn’t, many unions are "holding her endorsement hostage,” Bannon argued.

“They’re very angry at Hillary for sitting on the fence and not taking a stronger stance during the trade battle,” he said.

Labor leaders say they are keeping their options open. Trumka said it is “conceivable” that the AFL-CIO — the nation’s largest labor federation — would not endorse a candidate at all during the primaries.

Adding fuel to the fire, Trumka met with Biden last week in Washington, and will march with him in a Labor Day parade. He said the primary race is “still wide open."

“Biden’s a good friend — he’s been a champion of working people,” Trumka said this week during a breakfast with reporters. “He would be a good candidates. He would be a good president.”

The labor movement has also lavished praise on Clinton’s chief rival, Bernie Sanders.

"I think he’s connecting,” Trumka said at the breakfast. "He has a very unique and genuine way of talking about the most pressing issue in politics, and that’s inequality in America.”

But Democratic strategists say the praise for Sanders is a ploy by labor leaders to secure more support from Clinton.

“They want to squeeze out every bit of pro-labor policy they can from Hillary before they make an endorsement,” Bannon said.

Labor would also like stronger commitments from Clinton to not only fight for trade protections, but also push for an increase in the minimum wage and expand union membership, said Julian Zelizer, professor of history and public affairs at Princeton University.

“Their hesitation is creating leverage,” Zelizer said. “That way Hillary will feel a lot more pressure to speak up about their issues."

It’s all a charade, said Manley. Ultimately, he believes labor will get firmly behind Clinton.

"There is no alternative to Hillary Clinton,” he said. "As much as unions may admire Bernie Sanders, it’s still difficult to imagine him becoming president."