With 20 days to go until the Senate election, Rep. Ed Markey (D) and former Navy SEAL Gabriel Gomez (R) will square off in the first of three televised debates Wednesday night. There's plenty to watch and lots of ground for the two candidates to cover.

Gabriel Gomez (R), left, and Rep. Ed Markey (D). (AP)

Below are the four biggest things to keep an eye on. What did we miss? The comments section awaits your input!

1) Gomez's debut: This is Gomez's first campaign. He'll need to prove he can go toe to toe in a live debate setting against Markey, an experienced pol who has been in Congress since the 1970s. Sure, Gomez debated opponents in the primary. But never have the stakes been as high. For Gomez, it will be all about setting an early tone for the voters tuning in who don't know much about him. If he looks sharp and polished out of the gate, that will be good news for Republicans. If he stumbles, or fails to parry if/when Markey strikes, it will be bad news for the GOP. This is only the first of three debates, so Gomez doesn't have to pack everything into one. His focus needs to be on talking about his biography and introducing himself before Markey defines him.

2) How aggressive will Markey be? The Democrat's campaign has aggressively sought to paint Gomez as an out-of-touch Republican who is far too conservative for the state. On the eve of the debate, Markey released a rough ad hitting Gomez on abortion, social security and guns. Of course, negative ads and campaign attacks don't mean that Markey will be an attack dog in the debate. But given the tension between these two candidates (Gomez once compared Markey to "pond scum"), things could get heated. It's worth recalling that in Markey's first debate against Rep. Stephen Lynch (D) after the campaigns paused in the primary following the Boston Marathon bombings, he seemed a bit flat-footed and appeared to be caught off guard by Lynch's broadsides. The next night, Markey adjusted, and turned in a much more aggressive performance. We've seen Markey both play it safe and turn up the heat. What he does in round one against Gomez will offer insight into his current campaign strategy.

3) National themes: Markey's campaign has sought to tie Gomez to national Republicans and the national GOP agenda. This is no surprise, considering the Bay State's strong Democratic tilt. The onus — in this debate, and more broadly in the campaign — will be on Gomez to convince voters he is not beholden to the national Republican interests. Gomez has said he plans to be a "pain in the butt" to GOP Senate leaders. For Gomez to pull off an upset, voters have to come away from this and subsequent debates agreeing with him. Meanwhile, Markey will have President Obama stumping for him next week, reinforcing his deep ties to the Democratic Party.

4) Houses and taxes: A tax deduction Gomez took on his home became an issue in the race last month. Meanwhile, past criticism from opponents that Markey doesn't spend enough time at his own home in Malden has given Gomez fodder for criticism on a similar topic if he wants to use it. Then there is the issue of the two candidates' tax returns, which could also surface as points of contention. There's no guarantee any of this will come up, especially since candidates tend to save big hits for later debates. But if it does, get ready for some fireworks.