Tom Tancredo has no credibility as a presidential candidate. He seems to be a genuine xenophobe and a dangerous militant Islamophobe in particular. That said, I think people are going overboard in condemning his bomb-in-a-shopping-mall ad for the Iowa market. You can see it on this blog with predictable scolding commentary.
I have no intention of defending Tancredo's message or his fears, but I think that liberals and leftists have gone too far in their reflexive hostility toward the "politics of fear." The usual complaint is that Republicans try to scare voters out of any proportion to existing dangers and thus simultaneously convince the same voters that the GOP alone is vigilant and tough enough to defend us from every threat. I agree that Republicans exaggerate the existing threat, but the liberal reaction comes close, as I see it, to forbidding people from claiming that threats uncomfirmed by liberals even exist.
Tom Tancredo most likely believes, sincerely and hysterically, that the threat he portrays in the commercial is real. Others believe sincerely that "Islamofascism," or whatever they want to call it, poses an "existential" threat to the United States. Others yet believe on the basis of observation, accurate or not, that the U.S. isn't doing enough to prevent terrorists from entering the country. They all have as much right to raise the issue and sound their warnings as liberals have to refute them.
I don't think it's cheating in some sense that liberals use for Republicans or anyone else to warn of dangers they perceive. If anything, it may be a useful service, for should such threats be so self-evidently absent as liberals assert, the mere advertisement of such baseless fear should discredit a candidate at first sight. Liberals, however, expect the opposite to happen, because they can't help presuming that voters are infinitely gullible. That's why they live in perpetual terror of offending Rush Limbaugh or Bill O'Reilly, because they presume that hosts or herds of voters take every word of these talkers as gospel and act accordingly at the polls. Even after retaking Congress, it hasn't sunk in for these liberals that all they really need to do is answer the right wing yakkers in kind (as Keith Olberman does, for instance) to break their vaunted spell on the public. Of course, if liberals actually believe that the people themselves are incurably stupid and helplessly susceptible to Republican "fearmongering," their future defeats should not surprise them.