The deficit-reduction speech President Obama delivered from the Rose Garden on Monday underscores the sharp strategic pivot he and his administration have made in the wake of the debt-ceiling negotiations.

Call it lessons learned the hard way, or the necessary readjustment by a politician, but the Obama who spoke on Monday was in a far different place politically and stylistically from the president who tried to pull off a grand bargain with House Speaker John Boehner, R-Ohio, in July and August.

Obama is a politician whose first instincts always have been to try to find ways to entice, cajole, reason with or otherwise produce cross-party consensus. On Monday, he continued a transition toward greater partisanship that began with his speech to a joint session of Congress two weeks ago.

Rarely has this president been as blunt in his challenges to the other party as he was on Monday. Rarely has he been so willing to draw lines in the sand.

Obama has gone from a president who talked openly about his willingness to rile his own party by making concessions on entitlements to get a deal with the Republicans to a politician determined to reconnect with his base as the two parties head into an election campaign in which the stakes could not be higher.

Monday’s speech was another sign that many of the working assumptions Obama and his advisers took into the debt-ceiling talks have been replaced — some because of practical necessity, but more fundamentally because of political necessities.

With his own political standing weakened and his base in near-revolt, the president may have had no choice other than to reappraise both his economic and his political strategy.

Gone is any illusion that he and Boehner can really make a deal along the lines discussed during the summer — a deal that would take a serious bite out of entitlements, particularly Medicare, and include some new taxes. Gone too is his willingness to offer anything that would make his base even more estranged that it already is. Instead, Obama is under pressure to produce a program that can produce jobs and to reassert his standing as the leader of Democrats.

But with Boehner declaring taxes off the table in his speech last week, the president was left with little choice. He countered that the only fair way to reduce the long-term deficit is with a combination of taxes and spending cuts.

Perhaps this is just a different opening bid, the terms of which could change if the negotiations inside the supercommittee in Congress look like they are making genuine progress. But it seems less likely after the speech Monday.

With more force than at any time in his presidency, Obama pressed his argument of mutual sacrifice, with a significant share of it aimed at the wealthiest Americans. His call for an end to the Bush tax cuts for the wealthiest Americans and a millionaires’ tax on top was greeted by charges of class warfare from his Republican critics.

“This is not class warfare,” he said. “It’s math.”

If instant reactions are any indication, Obama’s speech helped reassure some Democrats that he is one of them and he will fight for the values they hold most dear. Daniel Mintz, the campaign director of the MoveOn.org, an organization often at odds with the president, issued a statement of praise.

“For months, hundreds of thousands of members of the American Dream Movement have been urging Washington to focus on creating jobs and making our tax system work for all Americans, not just the super rich,” Mintz said. “Today, we’re glad to see this message reach the White House.”

This was a speech guaranteed to draw criticism from Republicans, and it came quickly. Senate Minority Leader Mitch McConnell, R-Ky., offered this: “Veto threats, a massive tax hike, phantom savings and punting on entitlement reform is not a recipe for economic or job growth — or even meaningful deficit reduction. The good news is that the Joint Committee is taking this issue far more seriously than the White House.”

It may be a measure of the president’s weakness that he has backed off significantly on changes in entitlements. He excluded Social Security from any changes in his new proposal, with administration officials arguing that it should be dealt with separately and apart from the current round of deficit reduction.

More significant was his backing down on Medicare. During the negotiations with Boehner and Republicans, Obama was willing to agree to a rise in the Medicare eligibility age from 65 to 67. There was no mention of that Monday. Instead he returned to the kind of tough language he used last spring when he took on House Budget Committee Chairman Paul Ryan’s budget and in particular Ryan’s proposed changes to Medicare.

Obama’s speech was obviously anything but a blueprint designed to produce bipartisan consensus. For now, those days are over. Instead, the president has decided he must win the battle for public opinion in the debate between his vision and that of the Republicans if he hopes to win a second term in office.

He believes the American people are with him on the broad outlines and values he espoused Monday. The campaign that began on Sept. 8 in the House chamber continued Monday in the Rose Garden, with a weakened president fighting to win that battle.