INDIANAPOLIS (AP) — Indiana Rep. Luke Messer announced Wednesday that he will challenge Sen. Joe Donnelly, who is considered one of the most vulnerable Democrats up for re-election next year.

The GOP congressman, who represents Vice President Mike Pence's former southeastern Indiana district, took to Twitter to make the announcement. "We're in," he wrote in a post that also featured a campaign slogan: "I Like Luke."

Republicans and other affiliated outside groups have signaled that they are willing to spend vast sums to retake the seat that was long held by former Sen. Dick Lugar. But already the GOP primary is shaping up to be a brutal affair that could lead to months of Republicans fighting each other more than the incumbent Donnelly.

Messer and fellow GOP Rep. Todd Rokita — who is also preparing a bid — for weeks have traded insults and accusations, with both suggesting that the other is "unhinged."

In a recent email blast to supporters, Messer said he was "tired of Todd Rokita lying about my family." That came after Rokita raised questions about Messer's decision to relocate his family to suburban Washington D.C. after his 2012 election. Rokita also questioned a lucrative contract Messer's wife Jennifer holds with an Indianapolis suburb for part-time legal work she does from home.

The Associated Press first reported in May about Jennifer Messer's contract with the city of Fishers, which pays her $240,000 a year to work as a part-time contract attorney, a sum that is vastly more than either of the city's two staff attorneys are paid — or many other government lawyers in the state.

"Todd's attacks on me are especially remarkable because we've known each other for twenty-nine years," Messer wrote. "What's changed? Now Todd Rokita wants to run for the U.S. Senate, and it's clear that he will do and say almost anything to advance his political career."

Rokita responded with a missive of his own, writing, "When someone politically connected is pocketing $240,000 per year in taxpayer money for part-time work, Hoosiers have a right to ask questions."

Messer has already lined support from many of the state's most influential Republicans, including the vice president's brother Greg Pence, who is serving as campaign finance chairman. Messer is also supported by key Pence fundraiser Marty Obst and Indiana Republican National Committeeman John Hammond III, among others.

Responding to the announcement, Donnelly campaign manager Peter Hanscom played up the Democrat's record of bipartisanship and said "there's still only one person on next year's Senate ballot who is deeply connected to the Hoosier electorate and has a proven track record of working across party lines to deliver for Hoosiers, and it's Joe Donnelly."

Meanwhile, Messer's campaign was more focused on attacks from Rokita.

"I think when those people see some of the nonsense and negativity that comes from the Todd Rokita campaign they are going to be turned off," said Messer campaign spokesman Jason Kneeland.