The question now becomes, will he ever?
The White House and Senate Majority Leader Harry Reid are shy of the 60 votes they need to move the bipartisan compromise bill on background checks for gun sales.
Vice President Joe Biden has been personally calling senators to urge them to support the measure, Democratic aides say.
Majority Whip Dick Durbin (D-Ill.) began a whip count on Monday, Democratic aides said, and Biden has been pressuring his fellow Democrats to fall in line. Biden's office did not return an email seeking comment.