Michael Gerson here gets it right: Republican/conservative criticism of candidate Obama must be carefully-thought-out:
"Barack Obama will not be defeated by taunts about his middle name, which gain his juvenile persecutors all the sympathy of a schoolyard bully. He will not be defeated by sinister interpretations of his hypnotic popularity -- people generally (and unsurprisingly) are attracted to the handsome, genial and eloquent. And in a change election, Obama will not be defeated because he seems inexperienced -- his freshness is actually a qualification."
He won't be defeated by simply calling him a liberal or leftist, either.
But read Gerson's entire piece: what he does is to go on to show how Obama's campaign pledge to meet "without preconditions" with dictators such as Hugo Chavez or Mahmoud Ahmadinejad could turn out to be disastrous decisions, and could boomerang decisively against the national security interests of the United States. Principled conservative candidates can easily, and successfully, point these kinds of things out.
But such conservatives must be principled, and consistent, themselves.