Maybe it’s the magazine-model looks, the National Guard uniform, or the image he has crafted with his green pickup truck and brown barn coat.
Maybe it’s more substantive: his antitax talk, the Republican counterweight he offers in an otherwise all-Democratic delegation, or his success getting President Obama to sign some of his bills into law.
Maybe it’s his recent run of publicity. He has released a radio ad saluting St. Patrick’s Day and videos recapping his Opening Day visit to Fenway Park and welcoming his dogs to his South Boston headquarters.
Whatever the reason, US Senator Scott Brown is a likable sort, a Republican who even some Massachusetts Democrats supported when he won a special election in January 2010.
As he runs for reelection, though, one question confronting him is whether likability - and his constant effort to reinforce it in a myriad of substantive and more superficial ways - is enough to propel him toward victory.
He is facing a challenge from Democrat Elizabeth Warren, and Brown, his staff, and the Massachusetts Republican Party have done all they can do to portray her as unlikable as possible.
Beyond pointing out policy differences, they have alternately labeled her an “elitist hypocrite,’’ a “militant liberal,’’ or simply “professor,’’ suggesting the Harvard Law School teacher lacks any of his common touch.
Most recently, a GOP spokeswoman derided Warren for not following Brown’s example on Friday and wearing a Boston Red Sox jacket to Fenway Park for the ballpark’s 100th anniversary.
The goal is clear-cut: to define Warren negatively before she can define herself positively.
But the endgame may be more of a challenge than that.
A recent Globe poll showed that Brown is already defined, and likability is already factored into his support.
Warren, despite running almost evenly with Brown among voters with an opinion of the two, is still unknown to a sizable number of voters.
If she can succeed in presenting herself positively to those undecided voters, she has more room for growth in her overall support - regardless of how likable Brown is.
The Globe poll, conducted a month ago, showed that the incumbent senator had the support of 37 percent of likely voters, more than the 35 percent who backed Warren but within the survey’s margin of error.
A sizable 26 percent of those surveyed said they remained undecided.
Brown soared, though, when voters were asked a straight-up question: Which candidate is most likable?
He was picked by 57 percent of respondents, compared with 23 percent for Warren.
In terms of favorability, a more common poll index, both Brown and Warren performed well.
Fifty-four percent said they had a favorable opinion of him, versus 29 percent who had an unfavorable impression. Only 7 percent had no opinion.
For Warren, 47 percent had a favorable opinion of her, versus 23 percent who had an unfavorable impression. Yet 25 percent said they had no opinion yet.
That illustrates that voters have fairly set opinions about Brown more than two years after he took office, but are still formulating an opinion of Warren eight months after she announced her candidacy.
With Democrats in this traditionally Democratic state expecting a larger turnout on their side with a presidential contest atop the ballot this November, she has a chance to pick up votes from people who - at least right now - know Brown but haven’t yet settled on voting for his reelection, according to the Globe poll.
In 2008, US Senator Gordon Smith, a Republican popular in Oregon, another left-leaning state, was nonetheless defeated for a third term as Obama beat GOP rival John McCain there by a margin of 57 percent to 40 percent.
Andrew E. Smith, who heads the University of New Hampshire’s Survey Center and conducted the Globe’s poll, cautioned against reading too much into the results because it’s still early in the Brown-Warren race. More accurate predictions will come after summer turns to fall.
But the pollster also said Brown has to be careful to balance image with action.
“If people like you, they generally like you, and if you keep pushing that button, and it’s your only tune, it’s not one people like to keep hearing,’’ said Smith. “You have to give them something else, too.’’Glen Johnson can be reached at firstname.lastname@example.org. Follow him on Twitter @globeglen.