U.S. Sen. Scott Brown officially kicked off his re-election campaign Thursday, casting his chief Democratic opponent Elizabeth Warren as an ideologue and pledging to be an independent voice in a deeply partisan Congress.

Brown, speaking to a crowd of cheering supporters at Mechanics Hall in the Worcester, said he would continue to oppose the health care law signed by President Barack Obama and would fight against wasteful government spending.

The Massachusetts Republican timed his Thursday evening event to coincide with the anniversary of his special election win in 2010 that catapulted him into the office once held by the late Democratic Sen. Edward Kennedy.

Brown also criticized what he called "establishment candidates" who try to divide Americans for political gain.

"They'll wage class warfare, pitting one group of Americans against another," Brown said. "They will attack success, and our free enterprise system. They will use terms like 'us' and 'them.'"

Brown, who is facing a tough re-election campaign in a state that typically favors Democrats, has tried to position himself as the underdog in the race, despite his incumbent status and a campaign war chest more than twice as large as Warren.

Brown, who welcomed tea party support during his special election campaign two years ago, has gone on to break with his party on several key votes, including his support of a Democrat-backed overhaul of the nation's financial system. Brown's backing of a repeal of the military's "don't ask, don't tell" ban on gays serving openly also drew criticism.

Most recently Brown decided to support Obama's decision to name Richard Cordray as the nation's chief consumer watchdog despite the objections of most Senate Republicans.

Brown has pointed to those votes as proof of his independent streak.

"I told the voters that I wouldn't just be another loud, angry partisan," Brown said. "I don't worry about the party line. I don't get caught up in petty fights."

Brown's victory stunned Massachusetts Democrats, particularly because the seat had been held by nearly half a century by Kennedy. Brown famously used that connection when he declared that the seat was the "people's seat" and not the Kennedy seat.

Reclaiming that seat has become the top goal of Democrats. Many are placing their hopes on Warren, the consumer advocate and Harvard professor who helped launch the new federal Consumer Financial Protection Bureau.

While Brown enjoys an overall money advantage with $12.8 million in cash on hand, Warren collected more than $6 million as of Jan. 1, pulling in more than Brown during the final three months of 2011.

Warren's campaign said she had raised at least an additional $1.1 million Thursday during a "money bomb" fundraising effort from midnight to 9 p.m.

Warren was in Worcester on Thursday, shaking hands with diners at the Nu Cafe.

Responding to a portion of Brown's speech emailed to reporters ahead of the address, Warren brushed aside the criticism that she is trying to pit Americans against each other.

"This is the standard Republican play book right now," Warren said. "He is echoing the same things that every other Republican is saying every time a camera's turned on. I don't think the people of Massachusetts are going to be persuaded by that."

Warren added to her fundraiser total on Thursday with an online "money bomb" pegged to Brown's announcement. A money bomb allows supporters to pledge money via the Internet during a narrow window of time, with all the online "checks" cashed on the same day.

By Thursday, the amount pledged to Warren totaled more than $1 million. In the 2010 special election, Brown raised $1.3 million using a similar "money bomb" tactic.

While the race is widely expected to be the most expensive in state history, Warren and Brown are also taking the unusual step of trying to broker a deal to keep third party groups from sponsoring attack ads during the campaign.

Last week, Brown said he and Warren should sign a binding agreement to make hefty donations to charity if outside groups launch television, broadcast or online ads supporting their campaigns or attacking their rival.

Warren said she hoped to strike an even tougher agreement. She said she would like to see the two sign a joint agreement to be sent to all third party groups notifying them of the deal. She said the deal should specifically include radio ads and the candidates should ask broadcast outlets to help honor the deal.

Top staffers from both campaigns were expected to meet Friday to try to hammer out the details of an agreement.

What still isn't clear is how Brown or Warren could block the ads.

By their nature, the political action committees and outside groups that pay for the ads must work independently of the candidates and their campaigns. Federal election law explicitly bans candidates from coordinating with the committees on advertising.

Warren said it's still worth a shot.

"It's an effort to try to change the environment for third party groups," Warren said Thursday. "This is important and it's worth trying to do something to try to keep them out."

Warren still must win the state's Democratic primary in September. She is being challenged by other Democratic candidates including Marisa DeFranco, an immigration lawyer from Middleton, and James King, a lawyer from Dover.