Front-running Democratic presidential candidate Hillary Clinton and her top challengers, Sen. Bernie Sanders and former Maryland Gov. Martin O'Malley, will meet privately with the AFL-CIO's executive council at the end of July.

Union leaders will press the candidates on their stances, particularly on international trade, at the meeting in Silver Spring, Md., Reuters reported.

That will be a tricky issue for Clinton, who has tried to stay above both the recent congressional fight over Trade Promotion Authority legislation and the looming one over the Trans-Pacific Partnership, a 12-nation trade deal. Clinton has not taken definitive positions on either. Labor leaders strongly oppose both, and AFL-CIO President Richard Trumka has said trade will be crucial to 12 million-member labor federation's presidential endorsement.

Sanders, I-Vt., has consistently and loudly opposed President Obama's trade agenda. O'Malley opposes both as well.

"There is no middle ground, and the time for deliberations is drawing to a close. In the 2016 campaign, there will be no place to hide for those who aspire to lead America," Trumka said in an April speech at the AFL-CIO's Washington headquarters on trade and the election.

Asked after the speech if he was calling on Clinton to issue a clear statement on Trade Promotion Authority, Trumka told the Washington Examiner, he was referring to "all candidates." He added later that Clinton "would have to respond like every candidate."

Clinton, nevertheless, did not issue a clear statement. Her status as the front-runner far ahead of all other Democratic candidates in the polls has made liberal groups reluctant to criticize her. Sanders has gained in recent polls, though, and the prospect of Vice President Joe Biden entering the race means she may not be able to count on that advantage forever.

Trade is treacherous territory for Clinton. Her husband, former President Bill Clinton, used a version of Trade Promotion Authority to help secure passage of the North Atlantic Free Trade Agreement in 1994, but she voted against renewing the authority in 2002 when she was a New York senator.

Coming out in opposition of the Trans-Pacific Partnership would put Clinton at odds with the administration where she served as the top official on international relations and took part in the deal's negotiations. Supporting it would put her at odds with much of the party's base as she tries to lock down the party's presidential nomination with a minimum of drama.

She has tried to adopt a more skeptical position on trade in recent months, but has left open the possibility that she could support the administration's agenda.