WASHINGTON — His political credibility on the line, President Barack Obama testily defended his willingness to choose compromise over combat with Republicans on Tuesday, lecturing fellow Democrats not to be "sanctimonious" purists.

Sensitive to charges of caving on bedrock principles, he said he welcomed fights with the GOP ahead of his 2012 re-election bid.

"I will be happy to see the Republicans test whether or not I’m itching for a fight on a whole range of issue," Obama said. "I suspect they will find I am. And I think the American people will be on my side."

The subject was taxes, who would or wouldn’t keep Bush-era reductions come Jan. 1. But for Obama, barely a month after disastrous congressional losses to the Republicans, there was a lot more to it.

What emerged Tuesday was a portrait of a president determined to show he’s not a weak, irrelevant capitulator — the kind of image that, if it becomes part of a lasting narrative, could derail his presidency and re-election bid.

In the past few days, Obama has tried to recover from the midterm elections by showing deference to his opponents, angering allies in the process.

The key moment came Monday, when he announced a deal with Republicans that would extend tax cuts to all taxpayers for two years, after long insisting that upper income Americans did not need the help and the nation couldn’t afford it. Though he won a number of concessions from Republicans, congressional Democrats were left bristling.

Besides the tax deal, he also disappointed labor by calling last week for a freeze on federal wages. And he has insisted that the Senate take up a nuclear arms treaty ahead of other Democratic priorities.

It’s a template for a new Washington relationship after two years of relying on Democratic muscle to pass the health care overhaul and other of his signature initiatives.

For Obama, this political pirouette could be both risky and unruly, causing chaos within his own party while requiring the support of Republicans who are openly seeking to make him a one-term president.

"The president is confronted with a very difficult situation," House Majority Leader Steny Hoyer said Tuesday, describing the bitterness many Democrats voiced against the tax deal.

Obama’s news conference was meant not just to lobby for the agreement he made with Republicans but to lobby for himself as a leader who hasn’t lost sight of what he called the north star: "What is helping the American people live out their lives?"

Pressed on why he wasn’t able to keep from getting so boxed in, Obama called out Republicans for ideological rigidity on tax cuts for the rich ("This is their holy grail") and Democratic lawmakers for not acting earlier ("I would have liked to have seen a vote before the election").

Obama was so determined to show his toughness, in fact, that he compared Republicans on Tuesday to hostage-takers willing to do serious harm. To Americans.

In an echo of his 2008 campaign, Obama sought to define himself as a doer, not a partisan fighter. And he signaled a more incremental governing style, recalling the birth of Social Security and Medicare as far more modest proposals than they are now.

He argued that some of his critics on the left would prefer to "have the satisfaction of having a purist position and no victories for the American people."

"That can’t be the measure of what it means to be a Democrat," he said.

For Obama, the nation’s first black president and one who isn’t quick to bring up his race, his final defense of dealmaking was his most eye-opening.

"This country was founded on compromise," he said. "I couldn’t go though the front door at this country’s founding."

Obama insisted Tuesday that he still has plenty of fight left to confront Republicans and defend Democratic principles. The tax cut deal, he said, was essential to prevent a tax increase on all taxpayers and it bought him time to fight Republicans later on extending tax cuts to the wealthy.

"I’m as opposed to the high-end tax cuts today as I’ve been for years," he said. "When they expire in two years I will fight to end them."

Three days into his presidency in 2009, Obama told Republicans pushing their own ideas that there were consequences to the 2008 election. "At the end of the day, I won," he said.

For two years, that motivated Obama’s decision making.

The Nov. 2 elections, however, put the House in Republicans hands starting in January and trimmed the Democratic majority in the Senate.

Obama’s new approach echoes that of President Bill Clinton, who saw Democrats lose control of the House two years into his first term in 1994. Clinton, too, provoked Democrats by reaching agreements with Republicans, most notably on an overhaul of welfare laws that left liberals seething.

Eager to avoid a wholesale revolt, Obama has voiced support for top Democratic initiatives that his allies want to complete this year before Congress adjourns. But as he pressed for a tax agreement and a Senate vote on the START nuclear treaty, the likelihood of action on immigration and don’t ask, don’t tell are slipping away.

"There’s no question he has made a pivot," said Matt Bennett, a former Clinton aide and a vice president at the think tank Third Way. "He lives in a world profoundly different than he did before Nov. 2."

During his news conference, Obama’s bring-it-on defiance to all skeptics brought a smile from his spokesman, Robert Gibbs, who sat a few feet away from his boss. The Obama White House almost relishes the moment when people count them as down, if not out.

The president’s fiercest words came for members of his own party. He shot back at those concerned he was compromising too much. He compared the situation to the health care debate, when liberal Democrats complained that he caved in by failing to include a government-run insurance plan. He said they still miss the bigger picture.

"If that’s the standard by which we are measuring success or core principles, then let’s face it, we will never get anything done," he said. "People will have the satisfaction of having a purist position and no victories for the American people. And will be able to feel good about ourselves and sanctimonious about how pure our intentions are. … That can’t be the measure of what it means to be a Democrat."

And finally came a dare to fellow Democrats.

"Take a tally. Look at what I promised during the campaign. There’s not a single thing that I’ve said that I would do that I have not either done or tried to do. … To my Democratic friends, what I’d suggest is, let’s make sure that we understand this is a long game."