COLUMBIA, S.C. — Opening a tea party-backed forum in this first voting state in the South, presidential hopeful Michele Bachmann on Monday criticized President Barack Obama's understanding of the Constitution and pledged fidelity to the nation's founding document as she wooed the grassroots activists who could deliver her the GOP's nomination.

Bachmann said the 2012 presidential election would hinge on each candidate's understanding of the Constitution, which she called "that sacred document." Branding herself a "constitutional conservative," the former federal tax lawyer challenged Obama's understanding of his powers. She cited Obama's political and policy advisers, whom she called "czars," the Justice Department's decision not to appeal a court's overturning of a federal marriage law, and his immigration policies.

"These are areas where we see unconstitutionality," she said of Obama, a Harvard Law School graduate and former constitutional law lecturer at the University of Chicago.

Bachmann was the first speaker at an event that drew the top Republicans vying for their party's presidential nomination to this early voting state.

Former House Speaker Newt Gingrich played up the founding fathers' writings on liberties during his appearance: "These rights are inalienable. That means no politician, no bureaucrat, no judge can take that away from you."

Rep. Ron Paul of Texas, a favorite of the GOP's libertarian wing, decried government largesse: "People were supposed to carry guns, not bureaucrats." He also warned against a Washington that gives the Federal Reserve too much power.

"Our founders detested the ideas ... that allowed governments and central banks to spend money out of thin air," he said. "The counterfeiters are over there at the Federal Reserve."

With Labor Day marking the unofficial start to the 2012 campaign, the contenders were pitching themselves to tea partyers and the GOP base during an afternoon forum with Sen. Jim DeMint in his home state. The event was designed to probe the candidates on their views of spending, taxes and the Constitution — bedrock principles for the tea party activists whose rising clout is likely to shape the nominating process.

Ahead of the forum, Texas Gov. Rick Perry told a friendly, 400-person audience in Myrtle Beach that voters were the best term limits and they should look at his tenure as Texas' longest-serving governor.

"You are what term limits should be about," Perry told voters. He warned that if elected lawmakers were constantly leaving office, "you just embolden and empower the bureaucrats" who continue in their jobs beyond officials' terms.

Perry spoke at a town hall-style meeting before heading home to Texas in a last-minute schedule change to monitor raging wildfires.

"Gov. Perry called me personally, and with the wildfire situation ... he felt he needed to be back in Texas," DeMint told audience members before the televised forum was set to begin. "He wanted me to apologize to all of you for missing the day but he needed to be back in Texas. They've got a bad situation that's getting worse."

Even before the forum, a Bachmann-backing group was running television ads here criticizing Perry's record.

"Rick Perry doubled spending in a decade. This year, Rick Perry is spending more money than the state takes in, covering his deficits with record borrowing," the Keep Conservatives United ad says.

"There is an honest conservative — and she's not Rick Perry."

Perry aides dismissed the ad from the group he described as "Congresswoman Bachmann's front-group" as "patently and provably false."

If the ad underscored the rivalry already blooming between the pair, Bachmann's remarks seemed centered on Romney, who has seen his front-runner status weakened. Bachmann warned that Obama and Democrats' health care legislation was taking away freedoms and giving Washington abject power.

Left unsaid: Romney's plan in Massachusetts was a model for Democrats' national effort.

"They will become a dictator over our lives," she said of federal requirements included in the overhaul that requires Americans to have health insurance. Massachusetts requires a similar mandate.

"This is the foundation for socialized medicine. Make no mistake about it. It will change the face of this nation forever," she warned.

After keeping the tea party at arm's length most of this campaign, Romney appeared on Sunday at a Tea Party Express rally in New Hampshire and pitched himself as an outsider with less political experience than the rest of the Republican presidential field. He didn't use the words "tea party" during his speech and did little to shift his polished campaign speech.

Romney, who had initially planned to bypass the South Carolina forum, changed his schedule last week to join the event with DeMint, whose backing he enjoyed during his first presidential bid.

While DeMint is tremendously popular here in his home state and with his party's tea party faction, he isn't rushing to publicly pick a favorite this time and has suggested he might not back a candidate in the primary.

The appearances by the Republican candidates signaled an effort to draw in the libertarian-leaning voters who may have great sway in picking the nominee.

That's not to say wooing the tea party is without peril.

After Washington's debt showdown this summer, an Associated Press-GfK poll found that 46 percent of adults had an unfavorable view of the tea party, compared with 36 percent just after last November's election.

Associated Press writer Bruce Smith contributed to this report from Myrtle Beach, S.C.