Republican U.S. Sen. Bob Corker of Tennessee became the first incumbent this cycle to announce his retirement at the end of this Congress. His retirement also means that Majority Leader Mitch McConnell loses an important ally, and it opens up the chairmanship of the Foreign Relations Committee in the next Congress.

While Tennessee is a Republican-leaning state – Trump carried it by 26 points – it is also a place where Republicans are likely to host a competitive primary that pits more establishment Republicans against candidates who identify with the Trump brand of Republicanism.

There is already one announced candidate, Andrew Ogles, the former director of the state chapter of Americans for Prosperity, while former state Sen. Joe Carr, who challenged U.S. Lamar Alexander in the 2014 GOP primary, taking 41 percent of the vote, has been contemplating a bid. Knox County Mayor Tim Burchett and state Rep. Andy Holt have been mentioned as potential candidates. Republican members of the state’s congressional delegation are also likely to take a look at the race. In addition, there is a crowded field of candidates running for the gubernatorial nomination, and it is possible that one or two may explore the new opportunity that an open Senate seat provides.

On the Democratic side, attorney James Mackler, 44, has announced his candidacy. He is a veteran of the Iraq war where he flew Blackhawk helicopters in the Army’s 101st Airborne Division. He gets good reviews from some donors who have met with him in Washington. Other Democrats are likely to look at the race, including those in the congressional delegation. State House Minority Leader Craig Fitzhugh, who is currently seeking the Democratic gubernatorial nominee, and Chattanooga Mayor Andy Berke may also explore running for Senate.

Any Democrat would start the general election as an underdog, but national Democrats have good reasons to take this race seriously, not the least of which is that they need a third target if they hope to have even the remotest shot at the majority. The possibility that Republicans could hold a brutal primary that may produce a less than ideal nominee provides more incentive.

It will take some time for the fields on both sides to shake, but this race is worth watching. It moves to the Likely Republican column.