WASHINGTON (Reuters) - U.S. President Joe Biden returns to the campaign trail on Wednesday, rallying the key constituency of labor leaders, as some fellow Democrats continue to persuade him not to run again.

For the past 13 days, the 81-year-old Biden has been working to stem defections from Democratic lawmakers, donors and other allies worried he might lose to Republican Donald Trump, 78, after his halting June 27 debate performance.

Biden will join the AFL-CIO's executive council meeting in Washington on Wednesday to take questions from leaders of major U.S. labor unions and discuss "their shared commitment to defeating Donald Trump this November," the Biden campaign said.

Labor votes were key to Biden's win over Trump in competitive states, including Michigan, Pennsylvania and Nevada, in 2020.

Democrats in the U.S. Congress remain deeply divided over whether to fall in line behind Biden or to urge him to step aside because of persistent questions about his health and acuity. Biden has said he is fit to serve but understands the questions.

On Tuesday, Representative Mikie Sherrill of New Jersey became the seventh House Democrat to call on Biden publicly to drop out of the race.

Many more Democrats have expressed concern that Biden continuing at the top of the ticket could cost the party the White House and both houses of Congress in November.

But public defections remain a small segment of the 213 Democratic-aligned House of Representatives members, and the party's leadership continues to back Biden publicly. No members of the Senate have publicly said Biden should stand aside.

Biden, eager to change the story, has surrounded himself with communities of his staunchest supporters, including Black Democratic lawmakers and voters. His campaign has framed sticking with Biden as a return of the loyalty he has shown them through his half-century of public life.

Biden's first 2020 campaign rally, in 2019, was at a Pittsburgh union hall, and the president has made his thick-as-thieves alignment with Big Labor leaders a major pillar of his populist economic platform. Last September, he became the first sitting president to join a union strike when he met United Auto Workers asking for raises.

Labor leaders expected at Wednesday's meeting include the national presidents of 60 unions, representing 12.5 million Americans, Biden aides said.

Also this week, Biden has used the NATO summit as a global stage, with a forceful speech denouncing Russian President Vladimir Putin. On Wednesday, he will hold meetings with NATO leaders and then host a dinner for heads of state. The dinner would not normally draw much attention, but concerns over whether Biden can handle the demands of the presidency for another four years have put every Biden event in the spotlight.

Vice President Kamala Harris, who is the party's top alternative should Biden drop his candidacy, will speak to the Black Alpha Kappa Alpha Sorority Inc in Dallas after a Las Vegas campaign stop on Tuesday.

After the NATO summit ends, Biden will hit the road again, traveling to two of the competitive states, Michigan and Nevada, that he must sway to defeat Trump.

White House spokesperson Karine Jean-Pierre faced another salvo of questions from reporters about Biden's health on Tuesday. In a statement, the White House physician said Biden was not being treated for any neurological condition and had received a clean bill of health at his most recent physical examination in February.

(Reporting by Trevor Hunnicutt; Editing by Heather Timmons and Miral Fahmy)

By Trevor Hunnicutt