For most of the year, the Republican presidential race has been a listless affair. That changed suddenly Thursday night during a raucous debate that produced the sharpest exchanges of the campaign and signaled a new and more intense phase in the nomination battle.

Thursday’s two-hour debate bore almost no resemblance to a similar gathering in New Hampshire two months ago. If that was an example of timid candidates on good behavior, this was a case of nearly everyone needing to prove something and fearing they had little time to prove it.

Minnesota Rep. Michele Bachmann and former Minnesota governor Tim Pawlenty clashed repeatedly over each other’s records and qualifications. Pawlenty, not wanting to make the mistake he made in New Hampshire, challenged former Massachusetts governor Mitt Romney over health care.

But others sought to leave a mark as well. Former Pennsylvania senator Rick Santorum sparred with Texas Rep. Ron Paul over foreign policy. Former House speaker Newt Gingrich chastised moderator Chris Wallace of Fox News for asking about his struggling campaign. Former Utah governor Jon Huntsman Jr. sought to reinsert himself into the race after a shaky summer. Businessman Herman Cain hoped to erase memories of his weak performance in the earlier debate.

If this debate did not fundamentally change the shape of the race, it underscored that all of the candidates sense that change is coming — and they are anxious not to be left behind as the race heads into the fall.

The politicians debated at a crucial moment in the Republican race, with Texas Gov. Rick Perry ready to join the campaign with a bid that many strategists believe will pose an immediate threat to Romney’s front-running status. And Saturday’s Iowa straw poll could effectively knock out some candidates.

No one had more on the line than Pawlenty and Bachmann, who are on a collision course in the straw poll. The two have been sparring by long distance ever since the Minnesota congressman zoomed past the former governor in national and Iowa polls, and their disagreements boiled over Thursday in a series of tense exchanges.

Pawlenty played the role of aggressor, arguing that Bachmann’s record of accomplishment in Congress was “nonexistent.” But Bachmann more than held her ground, accusing Pawlenty of a series of policy positions as governor that made him seem more like President Obama than a conservative Republican.

From there, the rhetorical battle escalated, or perhaps descended, into sharper and sharper lines of attack that made palpable the apparent dislike between the two. “She said she’s got a titanium spine. It’s not her spine we’re worried about. It’s her record of results. If that’s your view of effective leadership with results, please stop, because you’re killing us,” he said, drawing an audible response from the audience.

Advisers to both candidates seemed pleased by the exchanges.

Those clashes, and the voluble exchanges between Paul and Santorum soaked up time and energy on the stage, with the result that Romney once again came out of the debate relatively unscathed. It was the second time that the frontrunner was mostly able to deliver his message on the economy, this time content to watch the fireworks around him.

But he did not emerge totally unscathed. Challenged on health care and his support for an individual mandate similar to that in the legislation signed by Obama last year, Romney parried with a defense that cited his support for the 10th Amendment and the power of states to make their own decisions. But Bachmann too jumped in, signaling what many Republicans have long predicted, that the issue could continue to dog Romney throughout the nomination battle.

Paul was a forceful presence on the stage, delivering his antiwar message with passion. He could be a spoiler Saturday, though a value of a surprise first-place straw poll finish likely is questionable.

Santorum, who at times seemed to disappear as others dominated the debate, is also at risk on Saturday and repeatedly was at pains to draw attention to himself and his morality-based campaign message.

More than anyone, Gingrich came alive on Thursday. The former speaker displayed the same brash style that marked his tenure as House GOP leader, and he clearly had the audience on his side when he took on his questioner. But it wasn’t clear how he might translate that into broader support from Republican voters.

Huntsman, making his debate debut, tried to tout himself as a business-friendly former chief executive and an authority on foreign policy. But he, too, had to work to make himself a central player, given the theatrics elsewhere on stage.

By the end of the month, much more will be known about the shape of the Republican race. But Thursday’s debate offered a preview of what is to come. With the stakes growing and the cast of characters changing, the Republicans face what could be a fierce battle for the nomination — one that will look far different than the campaign so far.