WASHINGTON — President Joe Biden’s decision to stand alongside United Auto Workers pickets on Tuesday on the 12th day of their strike against major carmakers underscores an allegiance to labor unions that appears to be unparalleled in presidential history.

       Experts in presidential and U.S. labor history say they cannot recall an instance when a sitting president has joined an ongoing strike, even during the tenures of the more ardent pro-union presidents such as Franklin Delano Roosevelt and Harry Truman. Theodore Roosevelt invited labor leaders alongside mine operators to the White House amid a historic coal strike in 1902, a decision that was seen at the time as a rare embrace of unions as Roosevelt tried to resolve the dispute.

       Lawmakers often appear at strikes to show solidarity with unions, and during his 2020 Democratic primary campaign, Biden and other presidential hopefuls joined a picket line of hundreds of casino workers in Las Vegas who were pushing for a contract with The Palms Casino Resort.