The news comes after a testy day on Capitol Hill, where House Democrats peppered Sanders with questions about his commitment to the Democratic Party, a party he first joined during the primary. Multiple people at the meeting told The Hill that lawmakers groaned when he said that his goal was "not to win elections" but to overhaul the political process.

But Wednesday also brought a major sign of warming between the two camps when Clinton laid out a debt-free college tuition proposal — a key issue Sanders has pushed during the primary but that Clinton had swatted away.

While the proposal wouldn't go as far as Sanders's plan, he said in a statement that he wanted to "take this opportunity to applaud Secretary Clinton for the very bold initiative she has just brought forth."