(CNN) Indiana GOP Sen. Mike Braun filed paperwork on Wednesday to run for governor, a move that will trigger an open seat race to replace him come 2024.
It's among the first major chess moves of the emerging Senate map -- one that, by the numbers, favors Republicans.
Of the 33 seats on the ballot, 23 are either held by Democrats or by independents who caucus with the party (Sens. Angus King of Maine and Bernie Sanders of Vermont.)
And of those 23 seats, there are at least five that seem likely targets for Republicans.
At the top of that list is the seat held by West Virginia Sen. Joe Manchin, who represents a state Donald Trump carried by 39 points in 2020.
GOP Rep. Alex Mooney has already announced his candidacy against Manchin and Gov. Jim Justice has said he is "very seriously considering" a Senate bid.
The second-most vulnerable seat for Democrats is in Montana, where Sen. Jon Tester is up for a fourth term in a state that Trump won by 16 points in 2020.
Rep. Matt Rosendale and Rep.-elect Ryan Zinke are both considered potential challengers to Tester.
Beyond those two seats, Republicans are likely to heavily target Ohio Sen. Sherrod Brown in a state that Trump won by eight points in 2020, as well as Sens. Jacky Rosen of Nevada and Kyrsten Sinema of Arizona.
Secondary targets could be Sen. Tammy Baldwin of Wisconsin and Debbie Stabenow of Michigan, although both have proven to be strong campaigners in their states.
Which is a lot of states where Democrats have to play defense!
What makes the map that much harder for the party is the fact that of the 10 Republican seats that are up, there are very few obvious targets. Perhaps the best opportunity for Democrats is in Florida, where Sen. Rick Scott would be up for reelection. But after the massive victories for Republicans in Florida this year, it's hard to see Democrats running a serious challenge to Scott.
While open seats tend to be more competitive, Indiana isn't an easy lift for Democrats. The state has trended more Republican in recent years and Trump won it by 16 points in 2020.
The Point: The Senate map looks very good for Republicans in 2024. But as 2022 shows, candidates and the campaigns they run matter.