Scott Brown has spent most of his Senate career with a target on his back. But his maverick voting record and fundraising prowess have positioned him to achieve the unthinkable: winning statewide reelection in Massachusetts as a Republican.

Brown has, so far, avoided drawing a serious challenger in his bid for a full, six-year term, despite the Bay State's traditionally Democratic hue.

"He has not been the caricature. Yes, he's made himself more electable by being more moderate," said Rep. Barney Frank (D-Mass.), who passed on a challenge to Brown. And Frank said he expects other members of the delegation to stay out of the race as well, in part due to Brown's strength.

"I do think it's very unlikely that a member of the delegation will run, and in that sense Brown's strength has discouraged candidacies," he said.

Brown has sought to project a centrist image since being elected to the Senate in a special election in January of 2010 to replace the late Sen. Edward Kennedy (D). He supported Democrats' financial regulatory reform bill last year, and was one of a handful of Republicans to vote in favor of eliminating the military's "Don't ask, don't tell" policy.

"Massachusetts is not Texas, so I think demonstrating some independence and not party-line votes is good for his survival," said Sen. John Cornyn (Texas), the chairman of Senate Republicans' campaign committee.

Indeed, Public Policy Polling (PPP), a Democratic firm, released a new survey on Tuesday showing Brown with a lead over all Democratic challengers. He bests Martha Coakley (D), the state attorney general whom he beat in 2010, and who polls most competitively against Brown, 49-40 percent, with 10 percent undecided.

Brown's advisers have sought to carefully manage how he's built his public profile; he embarked on a press tour earlier this year to promote a new book, in which he revealed that he was a victim of physical and sexual abuse as a child. And Brown announced that he'd asked to spend his mandatory two weeks of duty as a National Guardsman in Afghanistan this year. His political advisers cast him as an independent senator who's kept his head down and focused on state issues during a year and a half in Washington.

"Sen. Brown was elected to do a job, he takes that job seriously, and his focus right now is on working with people from both sides of the aisle to create jobs and promote fiscal responsibility in Washington," said Eric Fehrnstrom, a top adviser to both Brown's campaign and former Massachusetts Gov. Mitt Romney's (R) presidential bid.

Brown has also taken steps to bone up on his political chops. He established a political action committee in early June, and his reelection campaign's set a purported fundraising goal of around $25 million - enough to scare off a number of would-be Democratic challengers. He had more than $8 million in the bank at the end of March, and Brown's campaign will likely report even more cash on hand when his Federal Election Commission filing is due this month.

Several relatively unknown Democrats have entered the race, including Newton Mayor Setti Warren, state Rep. Tom Conroy and Alan Khazei, the founder of a civic nonprofit. But there's a sense that the party establishment in Washington might be looking for more of a heavyweight name to go up against Brown. Boston Mayor Thomas Menino (D) distilled the sentiment last month when he said that none of the declared Democrats in the race would be able to beat Brown.

Rep. Michael Capuano (D-Mass.) has said he'll decide this summer whether to run, and some Democrats have floated the name of Elizabeth Warren, the Harvard Law professor who's favored by President Obama to lead the new Consumer Financial Protection Bureau. 

Capuano's office wouldn't comment, and Frank, an author of the Wall Street reform bill that established the new bureau, said he'd rather have Warren run that new agency than run as a candidate for Senate.

In the meantime, Democrats in the Bay State are beginning to worry publicly that they'll squander their best opportunity to pick up a seat in 2012 because of the delay in fielding a top-tier challenger to Brown.

"If Democratic Party officials in D.C. have a preferred candidate, let us know who it is and get him or her up here to start doing the hard work it will take to win," wrote Doug Rubin, a former chief of staff to Gov. Deval Patrick (D), on Monday in the Boston Herald. "Democratic Party leaders in D.C. need to go all in, or get out of the way."

Sen. Patty Murray (Wash.), who heads the Democratic Senatorial Campaign Committee and said she was unfamiliar with the piece, suggested she was comfortable with the state of the race.

"We are in a place this second where I feel very comfortable working to get someone running," she said. "We'll have a candidate."

"I think right now in Massachusetts we're suffering an embarrassment of riches when it comes to our Democratic field," said Kevin Franck, a spokesman for the Massachusetts Democratic Party. Franck said the field "very well could expand."

While he's staked out a relatively centrist record in the Senate, Sen. Brown has so far escaped major scrutiny from the Tea Party, which was galvanized in part due to its enthusiastic support for his candidacy. Sixty-three percent of self-described "very conservative" voters in Massachusetts said in the PPP poll that they had a favorable opinion of Brown.

Frank said he still thinks there's plenty of time for a solid Democrat to enter the race, and Brown's biggest enemy, in the interim, could be himself. He stumbled last month on whether he'd vote for House Republicans' 2012 budget, which includes controversial changes to Medicare. After having appeared to express support for the plan, he ultimately declared his opposition - after weathering some bruising attacks from Democrats.

He also drew guffaws by proclaiming that he'd seen pictures of the deceased corpse of Osama bin Laden, though Brown later acknowledged that he'd been duped by forgeries. Democrats gladly pounced, asking whether he'd travel to CIA headquarters to see the real photos of the dead bin Laden.

And Brown could easily find himself without the full attention of some of the advisers who'd helped him chart his way to victory in his initial special-election contest against Coakley. Romney's Boston-based operatives have served also as Brown's political support team, but now that the former Massachusetts governor's presidential campaign is under way, Brown's effort will be left to other staff.

"I think that the downside for Scott Brown is that his advisers are leaving him to focus on what they see as a more exciting race," Franck said.