The coming establishment Republican ad assault on Donald Trump will likely focus on one main charge: He's not a conservative. Most of Trump's critics on the right have made that point at one time or another, and a preview of future attacks — the anti-Trump ads run by the Club for Growth political action committee in Iowa in September — focused on Trump's conservative apostasy.

"Which presidential candidate supports higher taxes, national health care, and the Wall Street bailout?" the ad asked. "It's Donald Trump." The spot went on to say that Trump "has a record, and it's very liberal" and that he is "playing us [conservatives] for chumps."

Which leads to the question: Will a campaign slamming Trump as a non-conservative actually work in the context of today's Republican race? There's evidence to suggest it won't.

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The problem is that an ad accusing Trump of not being a conservative will appeal almost exclusively to GOP voters who are strongly conservative. But those voters are mostly already supporting other candidates. Trump's base of support lies elsewhere, and might end up largely unaffected by a he's-not-one-of-us ad campaign.

The short version of the problem could be this: An attack ad says Trump is not a conservative. Trump supporters — and other possible GOP voters, as well — say, that's OK, we're not conservatives, either.

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