To the chagrin of many in his own party, President Obama has almost always been willing to negotiate — on taxes, on spending, even on the social safety net.

But in the current fights over a potential government shutdown and the debt limit, the Democratic president has decided where he will draw the line.

On Thursday, Obama categorically rejected the Republican demand that he roll back the Affordable Care Act in order to keep the government open and also refused to entertain negotiations on raising the debt ceiling.

Both positions, a centerpiece of his strategy this fall, are striking for a president who has made his willingness to meet the opposition “more than halfway” a defining characteristic of his political persona. Obama gave in on a key feature of his health-care plan in 2010 — a public insurance option — and engaged in a lengthy negotiation over the debt ceiling in 2011.

But in his remarks Thursday, Obama said Republicans were trying “to blackmail” him over the health-care law and pledged that he would have none of it.

Countdown to the (possible) shutdown

“Some have threatened a government shutdown if they can’t shut down this law,” Obama said at an event at Prince George’s Community College in Largo. “Others have actually threatened an economic shutdown by refusing to pay America’s bills if they can’t delay the law. That’s not going to happen as long as I’m president.”

He added: “I will not negotiate on anything when it comes to the full faith and credit of the United States of America.”

But the approach exposes the president to significant risks. On the debt ceiling, Obama could be blamed for refusing to negotiate if the nation defaults and the economy falls into a tailspin. Republicans argue that they would be seen as the ones extending a hand to the other side and would blame the president for declining a deal.

And in refusing to negotiate over the Affordable Care Act — colloquially known as Obamacare — he is standing his ground over an unpopular law that is constantly facing new challenges, both technical and political.

On Thursday, the administration announced that it would delay a part of the law affecting small businesses. Meanwhile, Sen. Joe Manchin III (W.Va.) became the first Democrat to endorse a delay in the individual mandate, the provision that requires most Americans to obtain health insurance and that Republicans are seeking to postpone.

“I think the American people are on our side in saying that the president should come to the table and negotiate,” Rep. Peter T. King (R-N.Y.) said Thursday on MSNBC. “I think we can have meaningful negotiations as has been done in the past over the debt ceiling. We are on firmer ground and also I think there is a lot more progress that can be made.”

But Democrats have embraced Obama’s approach to the fiscal fight this fall after many recoiled at the compromises he supported in his first term.

In particular, many Democrats see Obama’s decision in 2011 to negotiate with Republicans over the debt limit as a profound mistake. Obama agreed to the deep spending cuts known as sequestration in order to get Republicans to increase the debt limit — cuts that are now chipping away at all his economic priorities.

“I think we got burned, and I think the president learned from that. There was not the clarity from the White House that we would not negotiate from the debt ceiling,” said Rep. Peter Welch (D-Vt.). “He made the agreement on the sequester that haunts us to this day, and frankly a lot of us thought that was a grave mistake.”

Welch said, however, that Obama’s new stance is a welcome change. “He’s been clear, direct and insistent,” he said. “It’s been reassuring to Democrats.”

Republicans argue that Obama is being disingenuous by refusing to negotiate. They note that lawmakers have often demanded concessions from the White House in order to raise the borrowing limit, which allows the government to pay for spending already approved by Congress.

But White House officials say Obama has no choice but to reject any discussion on the debt limit, saying it’s the first time that the minority party has tried to force massive changes to public policy by threatening a default of the federal debt.

White House senior adviser Dan Pfeiffer likened House Republicans to “people with a bomb strapped to their chest” in remarks to CNN. And White House press secretary Jay Carney told reporters that “it would be irresponsible” to allow the debt limit to become “part of a political extortion game.”

White House officials insist they’re open to negotiating over the nation’s balance of taxes and spending but just won’t make concessions in exchange for demands over the debt ceiling. Congressional Democrats argue that it’s Republicans who have been uninterested in compromise.

“We have tried for months to sit down at the table with Republicans and negotiate on the budget,” said Rep. Chris Van Hollen (Md.), ranking Democrat on the House Budget Committee. “They have absolutely refused.”

But Republicans say Obama’s actions have led to greater brinkmanship, pointing out that the president called House Speaker John A. Boehner (R-Ohio) last Friday simply to inform him that there would be no negotiations over the debt limit.

“We ask the president to call again, but this time, don’t call to say he won’t talk,” said Rep. Kevin McCarthy (R-Calif.) “Do exactly what the American people ask: Sit down and work together so that we can get this country moving one more time.”

Scott Wilson contributed to this report.