Often, it wasn’t just Hagel’s thinking that proved unpopular. It was his way of doing business.

“Chuck always marched to his own drum when he was in the Senate. Even by Senate terms, he tended to be very much a freelancer. He ended up rubbing some people the wrong way by doing that,” said former Sen. Judd Gregg (R-N.H.), an early first-term Obama choice to lead the Commerce Department who withdrew because of policy differences…

Few on Capitol Hill are predicting that Hagel will be defeated — Democrats hold 55 seats and Hagel has a handful of GOP friends left in the Senate — but Democrats privately acknowledge it could be a bloody battle. Only 25 of the 45 Republican sitting senators ever served with Hagel, who retired in 2009 after two terms. Of the 33 senators who made tribute speeches to Hagel when he left, only four are Republicans who still serve in the Senate.