Voters in Indiana cast their ballots on Tuesday in a Republican senate primary election with three well-funded competitors aggressively attacking each other over who is most closely aligned with President Trump.

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The winner will face the Democratic incumbent, Senator Joe Donnelly, in the general election in November.

Mr. Donnelly is believed to be one of this election year’s most vulnerable Democrats. But some Republicans worry that the primary campaign’s vitriol actually helps Mr. Donnelly and threatens a Republican win in November.

Here’s what to watch for in Indiana’s primary elections.