If you watch or listen to an ad for Senator Scott Brown in Massachusetts, chances are you'll hear at least one of the following terms; bipartisanship, compromise, Red Sox.

One term you might not hear: Republican.

That's because for Brown, the Republican incumbent in the closely watched Massachusetts Senate race, his party identity could put him at an inherent disadvantage in the solidly blue Bay State.

As of February 2012, the voter enrollment figures from the Massachusetts Secretary of State showed that out of the total 4-million-plus registered voters, just 466,431 were registered Republicans - about 11 percent. Registered Democrats outnumber them by three to one - 1,475,879 are listed by the secretary's office, an inherent advantage for Democratic challenger Elizabeth Warren.

As in other states, though, the largest group of registered voters in the Commonwealth of Massachusetts doesn't identify with either party; they're independent (or "unenrolled," as the state terms them), and there are 2,145,108 on voter rolls, about 52 percent of registered voters.

That's where the messaging in the Massachusetts Senate race takes its shape on both sides, particularly for Brown.

It's a given for both candidates that claiming those independent voters will be the key victory, and to that end both of them have sought to claim center ground. Brown has touted his record of bipartisanship in Congress, Warren has highlighted her history of fighting for the middle class. But in a presidential election year, where turnout is largely driven by the race between Obama and Romney, Brown will face a greater challenge. Political operatives say he needs to try and tread center-left, to appeal to Obama cross-over voters.

Reaching out to women voters - a voting bloc that, as a whole, tends to lean Democratic - appears to be a big part of the Brown campaign strategy. The campaign released two TV ads this week which both feature his wife, reporter Gail Huff. In the ads, Huff talks about Brown's support for her and her career.

"I was a reporter on Boston TV for many years. I don't know how many husbands would want their wives getting up at 1:30 in the morning to go to work. Scott did all the morning routine: get the girls up, get them fed, get them dressed, get them off to school," Huff says in an ad titled "Dad."

"He's always been very, very sure about the women in his life to have their own lives. He is by far the most understanding of women probably of any man I know," Huff says in the second ad, titled "Husband."

As Brown and Warren seek to distinguish themselves and appeal to Massachusetts voters, their respective campaigns have a unique messaging advantage - and disadvantage - compared to many of the other major Senate races this cycle: no outside spending groups.

In January Brown and Warren signed the "Peoples' Pledge" - an agreement that seeks to limit outside involvement in the race. According to the agreement, if outside spending groups run an ad promoting a particular candidate, that candidate must donate 50 percent of that ad's cost to a charity selected by the opposing candidate. So far, the pledge has held up, even through the controversy surrounding Elizabeth Warren's Native American self-identification.

Having only their opposition to contend with, the pledge ultimately gives candidates a better opportunity to control the image painted around their candidacy. However, the pledge also leaves the task of going negative to the candidates - forcing them to make the decision about just how much they want to dirty their hands. And of course, it puts a stronger emphasis on their own fundraising.

That last part shouldn't pose too much of a challenge for either candidate, however. More money has been raised for the Massachusetts Senate race then any other Senate contest this cycle - over $33 million, with Warren outpacing Brown, $15 million to $11 million, according to the Center for Responsive Politics. Brown has an advantage in terms of cash on hand - his campaign reports having almost $15 million in its war chest, as compared to Warren's nearly $11 million.

Polling shows Brown and Warren in a virtual dead heat, with five months to go until election day.