Colorado Sen. Cory Gardner is arguably the most endangered Republican senator running for re-election in 2020. He is one of two GOP senators defending seats in states that lean Democratic and only narrowly won his race in 2014. Gardner could also face stiff competition — more than 10 Democratic challengers have already stepped forward for the opportunity to take him on.

Let’s start with a look at where Gardner stands and what that could mean for his re-election chances.

Ideologically-speaking, Gardner has occupied the middle lane of his party, which is good for a Republican running in a state that leans blue. But in the Senate, Gardner has voted in line with President Trump’s position more often than Colorado’s 2016 presidential result would suggest. This could be a problem for him in 2020, considering Gardner’s narrow victory in 2014 and that the Democratic nominee for president has won Colorado three times in a row.

This far out, there’s no public polling showing how Gardner would do against potential Democratic opponents, but a January poll found Gardner trailing a generic Democrat 38 percent to 46 percent. And if recent history is any judge, having to run as a Republican in a Democratic-leaning state at the same time that a presidential race is happening at the top of the ballot will be challenging for Gardner: Every state that had a Senate race in 2016 voted for the same party in both the presidential and Senate election, a first.

Traditionally, Gardner’s incumbency would be a major plus for his re-election chances, but there’s growing evidence that incumbency doesn’t matter as much as it once did. Nonetheless, in a tight race, a slight incumbency advantage could make a difference. It may have given Gardner’s fundraising a boost: His campaign announced that it raised $2 million in the first quarter of 2019 and has a total of $3.4 million on hand. Also, there’s no current sign of a primary challenger, which means that Gardner can save his resources for taking on his Democratic opponent.

It’s only April, but more than 10 Democrats have already gotten into the race. So far, no U.S. House member has opted to run, nor have any current or former statewide elected officials. So Democrats lack an A-list candidate. Still, there may already be a nominal front-runner at this point: former state Sen. Mike Johnston. Johnston, who raised his profile by running for governor in the 2018 Democratic primary, has grabbed headlines by bringing in $1.8 million in the first quarter of 2019, nearly the same haul as Gardner’s. Other notable Democratic contenders include former Obama administration official Dan Baer, former state House Speaker Andrew Romanoff and former U.S. Attorney John Walsh.

More candidates may still jump in, including two women: former state House Democratic leader Alice Madden, who is expected to launch a bid soon, and Colorado Secretary of State Jena Griswold. Griswold only just won her office in November, but she has already met with Senate Democrats about a possible bid. If no one else gets into the race, Griswold would be the only Democratic Senate candidate who has won a statewide election, which might give her a name-recognition advantage. Plus, Griswold and Madden could both have an edge in a crowded primary if the success that women experienced in the Democratic primaries during the 2018 cycle continues into 2020.

Regardless of who faces off in the general election, the Colorado race will most likely be crucial to deciding which party wins a Senate majority. The GOP currently has a 53-47 seat advantage, so Democrats need a net gain of three or four seats to take control, depending, of course, on the outcome of the presidential race (the vice president breaks ties in the Senate). A Republican victory in Colorado would all but guarantee a GOP majority, so this race is a must-win for Democrats. We’re a long way from the election — a lot could change between now and then — but mark Colorado as a must-watch Senate race in 2020.