In five-and-a-half weeks, Massachusetts voters will elect their next senator. The special election for the seat once held by Secretary of State John Kerry is so far shaping up to be a competitive contest between Rep. Ed Markey (D) and businessman and former Navy SEAL Gabriel Gomez (R). Wondering what to watch during the remainder of the race? Below are four big things keep an eye on.

(AP photo) Democratic Rep. Ed Markey (left) and Republican challenger Gabriel Gomez (right). (AP photo)

1. Can Democrats define Gomez? Democrats moved quickly out of the primary with an effort to cast Gomez as a far-too-conservative-for-the-state Republican who's financial decisions (more on that below) suggest he is out of touch with most Massachusetts voters. Even as the attacks have flown, a handful of polls since the primary have shown a competitive race between the candidates. The June 25 election is still more than a month away, and neither candidate has opened up an air assault against the other just yet. Democrats, having lived through the embarrassment of 2010 are resolved not to let a Republican Senate candidate sneak up on them again, which suggests they won't be shy about blitzing Gomez if they have to. Of course, it also will be interesting to see when Gomez goes after Markey on the air. The Republican is, after all, still the underdog in this race.

2. Can Gomez keep the money chase respectable? With the Democratic establishment squarely behind him from day one, Markey has been a financial juggernaut. He badly outspent Rep. Stephen Lynch in the Democratic primary, and had more than $4.6 million in his campaign account in early April, and the ability to keep the cash flowing. Gomez, who infused his primary campaign with six-figures worth of personal money, likely won't match Markey dollar for dollar when all is said and done. The questions moving ahead are 1) How much more of his own money is Gomez willing to spend? and 2) How much confidence can he win from GOP donors?

3. House money: Among other things, early Democratic attacks have honed in on a legal tax break Gomez took involving his Cohasset home deemed excessive by critics and an appraiser who says Gomez didn't pay for valuation of his home (Gomez's campaign says the fee has now been paid). It's all been an unwelcome distraction for the GOP nominee. So it will be interesting to see when/if the GOP begins full-throated attacks against Markey's dwelling. Critics have long charged that Markey spends too much time away from his Malden home, in favor of the suburban D.C. home he owns. It wasn't an issue in the Democratic primary -- which was largely cordial until the final week -- but it's worth keeping an eye on in the general election, especially since Republicans are eager to cast the congressman as a creature of Washington.

4. Will outside group ads be a factor? While Markey has signed onto a "People's Pledge" designed to curb the influence of outside groups, Gomez has not. So unlike the 2012 race in which both Scott Brown and Elizabeth Warren each signed the agreement, it's possible outside group advertising could become a factor in this race.