Indiana Rep. Luke Messer (R) is set to announce his long-expected Senate bid next month, an aide confirmed to The Hill.

If Messer makes it out of the primary, he'll face Sen. Joe Donnelly Joseph (Joe) Simon DonnellyTrump meets with potential Supreme Court pick Amy Coney Barrett at White House Names to watch as Trump picks Ginsburg replacement on Supreme Court Momentum growing among Republicans for Supreme Court vote before Election Day MORE (D), considered one of the most vulnerable Democratic senators in next year's midterm elections. President Trump won Indiana by 19 points.



Messer spokesman Jason Kneeland confirmed to The Hill that the lawmaker plans to officially announce his bid at his family's annual barbecue on Aug. 12. The congressman announced his plans on Twitter Wednesday with a brief tweet.

We're in!! See you at the picnic on August 12th. https://t.co/nmj6LhyMGh pic.twitter.com/QFVfSG5kVK — Luke Messer (@LukeMesserIN) July 26, 2017

Messer, a member of House leadership, won his seat in 2012. He previously served as a state lawmaker and as the Indiana GOP's executive director.

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He's made no secret of his interest in a Senate bid. Sources close to Messer told The Hill in December that he had already begun to eye the seat. Over the past few months, Messer has cobbled together an impressive slated of backers onto his finance committee, which includes Vice President Mike Pence Michael (Mike) Richard PenceGardner signals support for taking up Supreme Court nominee this year Biden leads Trump by 12 points among Catholic voters: poll GOP brushes back charges of hypocrisy in Supreme Court fight MORE's brother, Greg.

Fellow Rep. Todd Rokita (R-Ind.) is also expected to enter the race. The two lawmakers have been waging a shadow primary against each other for months, a sign that the impending primary could get ugly.

Hoosiers have described the race as more of a debate of style over substance. While there are few major issues where the two disagree, Messer's approach is seen as more congenial, while Rokita is more aggressive.

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Allies of both lawmakers have accused the other side of planting damaging stories in the media—a Politico report found that Rokita had spent campaign funds on a private plane, while an Associated Press story reported that Messer's wife made more than $500,000 while consulting for an Indiana city and living in Virginia.

Messer unloaded on Rokita over his attacks related to that story in a letter to supporters last week.

"For months, Todd Rokita has spread lies and half-truths about my family, claiming we are not Hoosiers and attacking my wife’s legal career," Messer wrote.

"It's clear that he will do and say almost anything to advance his political career. Apparently, truth or integrity won’t get in the way."

In return, Rokita's spokesman blasted the email as "pounding the table and feigning outrage" to distract from "the very real vulnerabilities that Donnelly and national Democrats will savagely exploit."

While Messer had attempted to make an early show of fundraising force with his fundraising team, Rokita significantly outraised him in the second quarter, a revelation that gave him a jolt of momentum in the shadow primary. Rokita raised $1 million during that period to Messer's almost $600,000. Rokita holds the slight edge in cash on hand, with about $2.3 million to about $2 million for Messer.

The winner of the primary will have one of the best shots for Republicans to knock off a Democratic incumbent. Donnelly has earned praise for his bipartisan leanings and his appeal to blue-collar voters that Democrats have struggled with, but his state voted overwhelmingly for Trump in 2016, the same year that Democratic titan and former Sen. Evan Bayh fell short in a comeback bid against Republican Rep. Todd Young Todd Christopher YoungSenate GOP eyes early exit Why the US should rely more on strategy, not sanctions Davis: The Hall of Shame for GOP senators who remain silent on Donald Trump MORE.

Donnelly has also been recently dogged by a damaging news report of his own, an Associated Press story that found his family company outsources jobs even as he criticized trade deals that outsourced jobs. Republicans have seized on that story to tar Donnelly and soften him up as their two likely primary candidates slug it out.

But Democrats seen an advantage in the heated GOP primary fight.

The brawl between Congressmen Messer and Rokita is already one of the nastiest and most divisive in the country — exposing their flaws and demonstrating that their campaigns are all about their own political self-interest, not the issues that matter to hardworking Hoosiers," said David Bergstein, the Democratic Senatorial Campaign Committee's spokesman, in a statement responding to Messer's announcement.

Scott Wong contributed reporting.