Republicans have confidence that whomever the party nominates, the candidate will have a strong chance of beating Donnelly. The senator is one of 10 Democratic incumbents running in states Trump won in 2016.
Donnelly, along with Democratic Sens. Joe Manchin of West Virginia, Heidi Heitkamp of North Dakota and Claire McCaskill of Missouri, is widely considered one of the senators most likely to get unseated in November. Flipping his seat could prove crucial to whether Republicans can keep or expand their narrow 51-seat majority in the Senate.
Since Trump became president, Donnelly has cast himself as one of the most bipartisan members of the Senate. He and other vulnerable Democrats have joined with Republicans on some votes, signaling the threat they face in opposing Trump too often.
He has voted with Trump's positions about 55 percent of the time — the fourth-highest among current Democrats in the Senate, according to FiveThirtyEight. Donnelly trails only Sen. Doug Jones, D-Ala., Manchin and Heitkamp by that metric.
Most recently, he broke with most of his party colleagues to support Secretary of State Mike Pompeo's confirmation and vote to roll back some Dodd-Frank financial industry regulations. He opposed the Republican plans to overhaul the U.S. tax and health-care systems last year, which earned him Trump's ire.
Donnelly has done enough to at least make a plurality of Indiana voters have a positive view of him, according to a Morning Consult survey in April. Forty-two percent of the state's registered voters approve of his job performance, versus 32 percent who disapprove, according to the poll.
Democrats see a possible advantage in the bitter GOP primary to challenge Donnelly. The Democratic Senatorial Campaign Committee sent an email this week that in part argued "brutal GOP primaries ... have already left candidates in key races across the country wounded and cash-strapped."
The Senate Democrats' campaign arm highlighted both the Indiana race and a West Virginia Republican Senate primary, which will also take place on Tuesday. Democrats have cited the personal attacks in Indiana's GOP race as a possible advantage for Donnelly.