This is the way the Working Families Party works in New York, in which it's legal for multiple parties to nominate the same candidate. Basically, they endorse the Democratic candidate if the candidate has sufficiently labor-friendly policies; otherwise they may refrain from endorsing or nominate a spoiler candidate; and they use this to pressure Democrats into supporting their platform. A few years ago they endorsed a Republican for state senate who had worked particularly hard to raise the minimum wage. I think there are other minor parties in New York that work the same way.