America’s “tea party” movement claimed its first big scalp with the announcement overnight that Florida’s governor Charlie Crist is giving up his attempt to win the Republican nomination for the US Senate, and will contest the November Senate election as an independent.

Crist, governor since 2007, has been trailing his conservative challenger Marc Rubio by substantial margins. Although his record is not particularly liberal, a number of things have made him a target for the tea partiers and the Republican base: his aggressive support for fair elections in 2008, his endorsement of president Obama’s stimulus package, and his (widely reported but not officially acknowledged) homosexuality.

It would probably take psychoanalysis to work out which of these (or other) factors is most important to the GOP’s conservatives, but this is a big step in their campaign to drive moderates from the party.

There are obvious parallels to the defeat of Democrat Senator Joe Lieberman in the 2006 Connecticut primary. Lieberman had alienated his party’s base by his dogmatic support of the war in Iraq, but although he narrowly lost the primary, he went on to win re-election fairly comfortably as an independent. He still caucuses with the Democrats, even though he endorsed Republican John McCain in the 2008 presidential election.

I supported the challenge to Lieberman, and I don’t blame Republicans for wanting to ensure that their senators are faithful to their party’s values. The problem is that the conservatives have such a narrow vision of what those values are that they risk permanently alienating large sections of the American public.

On the other hand, the tea partiers seem to be capable of moderation when it’s necessary: witness their support for Scott Brown, as liberal a Republican as they come, in January’s Senate by-election in Massachusetts. Even the conservatives know that tacking to the right is electoral suicide in Massachusetts. Time will tell whether it works in Florida.