The roots of his new toughness are rooted in the nature of his convincing November win over Mitt Romney. Obama was carried to the finish line by supporters after his epic flop at the Denver debate. That seeded in him a greater sense of confidence and deepened his resolve not to be rolled by a recalcitrant GOP, as he was during the bitter 2011 fight over the debt ceiling, according to interviews with staffers and friends for “The End of the Line,” an eBook published in collaboration between POLITICO and Random House.

After his 2008 win, he talked a lot about bipartisanship. This time he’s determined to squeeze it out of Republicans. He believes he owes that to the people who voted for him.

“There’s no doubt that he found this one to be sweeter than the last one,” said one of Obama’s top aides. “It was weighing on him how much was at stake, how much of his entire legacy was on the line. His legacy had not been determined by the previous four years; that wouldn’t matter to history. It was all about the outcome on Election Day.”

With that outcome now in the history books, the people around the president now see him as a Democratic Reagan, a resilient and popular figure who can unify the country — if only the dead-enders will give him a chance.