Because of the media hype people on the left tend to think of the tea party as a separate wing, full of nutters, when it's just a strong conservative movement with populist tendencies that lacking any solid structure and hierarchy tends to bring in the whack-jobs who also tend to appear on the news because they make good news, most tea partiers I know are middle class and middle aged regular voters. They've little core universal tenets beyond 'big government bad' and most of their members vote republican, just prefer Inhofe's over Snowe's. So really sending a tea-party affiliate money for their campaign is just strengthening their primary bid and if they win their general election fund. You might derail a right-wing victory, but not by vote-siphoning, rather by making moderates nervous to vote for them in the General Election. But I'd bet that a campaign ad by a tea-party person funded by leftists would lose far fewer voted for them (it would probably gain them votes) than giving that money to a leftist who will run an ad with it gaining them votes. I would say you'd be better off voting for the same person with pocketbook and ballot.

Morality wise, you should really affiliate to the party you want to win, or the party you most sympathize with who you think can win, and vote for them in the general and in the primaries. It's dishonest to vote for a different party in the primary, and would if done in large numbers only result in parties going out of their way to only offer one viable candidate so their voters could cross and vote for the other party to tamper with them and make themselves hardened against rival party primary tampering. Just keep to the spirit of the thing, vote for who you want to win the general election, donate to people you want to win the general election. Democracy is already rough enough in translating the will of the people into action without adding any more road bumps from people trying to game the system.