In the UK, if you lend your support to someone in a different party, you're out on your backside. This happened to Ken Livingstone in 2000, because he ran as an independent candidate for Mayor of London.
In 2004 Zell Miller, a Georgia Democrat, told people to vote for George Bush. Lyndon LaRouche, a rightwing conspiracist and all around rum character managed to contest the Democratic primaries (some kind person could probably think of some Republican examples).
I thought of an older example. John Tyler succeeded to the presidency after Harrison died. Tyler was a whig, and after a few months the whigs kicked him out.
Did they use a rule to do this? Does a similar rule exist in modern parties?
Are there any circumstances where someone has the authority to say 'you're not a Democrat/Republican any more'?