AFL-CIO President Richard Trumka issued a strong warning to Donald Trump Friday that the nation’s workers are ready to stop any attempt by the administration to trash the U.S. constitution.

He was reacting to announcements by Trump that he considers the mail-in voting process unacceptable and that he will not commit to a peaceful transfer of power should he lose the election. Trump has said, essentially, that only an election he wins would be one he would recognize.