David Keene, president of the American Conservative Union, warns that a McCain presidential run could lead to a Nader-style third party challenge from an anti-immigrant candidate:

Republican divisions also loom in the 2008 presidential race. Arizona Sen. John McCain has joined forces with Kennedy on a guest-worker plan that is anathema to some conservatives. Assuming he runs in ’08, McCain is likely to face some GOP opponents who oppose guest workers and favor the so-called “enforcement first” approach.

Keene, an organizer of last week’s conference, said the issue could hurt McCain’s quest for the nomination, but he voiced doubt that immigration would be a defining issue in the GOP contest. The bigger threat for Republicans, said Keene, is that the nomination of someone such as McCain could inspire a Tancredo-style or “Ross Perot-like” third-party candidate who siphons off a small percentage of conservatives upset about immigration.

And it seems just as likely that a McCain nomination could provoke a general third party “true conservative” challenge from, say, a Roy Moore-style social conservative. These scenarios should give pause to those who believe he would be such a strong candidate in the general election.