Indiana Rep. Luke Messer is formally announcing that he will challenge U.S. Sen. Joe Donnelly for his seat in 2018.
The Republican congressman previously formed a finance committee and was widely anticipated to run. But he waited until Wednesday morning to officially announce the bid on Twitter.
Donnelly is considered one of the most vulnerable Senate Democrats in 2018. Republicans have signaled they will spend considerable sums to retake the seat previously held by former Sen. Dick Lugar.
But already the GOP primary is looking like it could be brutal. Messer and fellow Rep. Todd Rokita have been critical of each other in recent weeks. A recent poll showed the two are in a virtual tie.
Rokita has yet to formally announce his candidacy but has hired key campaign staffers for an expected Senate bid. And all three candidates are raising campaign funds for the race.