You can see the Marion County Superior Court’s decision in Rust v. Morales and commentary here. More commentary from the Indianapolis Star. The law requires primary candidates either (1) voted in that party’s past two primary elections or (2) received approval to run from the county party chair. John Rust voted in the Democratic primary in 2012. The court found that the law runs afoul of the Anderson-Burdick balancing test as placing an undue burden on the right to vote; that it violates the 17th Amendment, which guarantees that “the people” vote for Senators; and that it violates other state provisions of law.
I think, at least on federal law, the holdings are likely mistaken. Rick P. last year highlighted a challenge to a similar law in Tennessee, and he rightly emphasized the party’s right to affiliate with candidates, and to exclude candidates it prefers not to associate with. Here, Indiana Republicans do not want Rust to appear as a primary candidate, and they appear entirely comfortable with the state legal system in place. It is strange, then, that the court so readily assumes the candidate has a right to forcibly associate with a political party as a candidate on the ballot when the party has a mechanism to associate with the candidate but no desire to do so. Indeed, in Newsom v. Golden, a challenge to the Tennessee law, I think the court got it right. But these associational cases in political primaries are not very easy (related problems arose in Utah in 2018 in the 10th Circuit here and here), and we’ll see how the case proceeds on appeal (as I mentioned, the court found the law flunked several different tests, so it’s not clear how it plays out).