“Wouldn’t it be contradictory to vote for a Democrat in the Republican primary?”
Not if the Dems are involved in an “Operation Chaos” kind of thing to manipulate GOP returns. They would get a Republican ballot, then vote for Kasich or Carson to bump those numbers, and detract from Trump. I believe this was done in NH.
From what I’ve read here, there and around and about, many Dems will vote for Trump because they like him. Notice that there have been no polls, that I have seen, that include ALL likely voters, when asking about all the candidates. Wait till those numbers are revealed. Trump’s supporters including large numbers of Dems, Indepentents, and people who never have voted, but plan to vote for Trump.