Story highlights Communications Workers of America, Congressional Progressive Caucus PAC endorse Hillary Clinton for president

Rep. Barbara Lee, a liberal California lawmaker, "confident" Clinton will unite the party

New York (CNN) Groups that supported Bernie Sanders over Hillary Clinton in the Democratic primary are starting to coalesce around the former secretary of state.

Ahead of Sanders' expected endorsement of Clinton in Portsmouth, New Hampshire, the Communications Workers of America and Congressional Progressive Caucus PAC endorsed Clinton for president on Monday. Also joining those groups: Rep. Barbara Lee, a liberal California lawmaker who refused to endorse Clinton earlier this year.

Clinton and Sanders, after months of at times acrimonious campaigning against each other, will stand on the same stage Tuesday, where the liberal senator is expected to end his campaign and back Clinton.

This trio of endorsements comes as Democratic voters, many of whom were split between Clinton and Sanders, are starting to unify around the former secretary of state.