"One area of contention in open primaries is “crossover” voting. It most often involves voters affiliated with one political party voting in the primary of another political party to influence that party’s nomination. For example, if a district routinely elects the Democratic nominee, Republican voters may vote in the Democratic party primary to attempt to influence the outcome. This could be a good-faith attempt to select a more conservative Democratic nominee who would be palatable to the Republican voters, or it could be sabotage, an attempt to nominate a weaker candidate who is easier to defeat in the general election." From
https://fairvote.org/archives/open-and-closed-primaries/