Nothing Donald Trump could say or do would drag him down in the polls.

But also, nothing the brash billionaire could say or do can prevent the slide that is coming.

The offensiveness of Trump's comments and quasi-policy proposals are their virtue, politically. His true weakness is that only a sliver of his supporters actually want him to be the Republican nominee.

A close reading of the polls helps show the shallowness of Trump's support. Trump's strongest arena — the national poll — is a nearly meaningless artifact this close to the primaries. In the earliest parts of the primary season, a national poll tells you only who is widely known and liked, not who will win. In the later parts of the primary season a national poll can be a very imperfect proxy for a multi-state poll.

But within two months of Iowa and New Hampshire, the only polls that really matter are the polls of Iowa and New Hampshire. Why? Because the results in Iowa and New Hampshire affect voters in the later states. Many candidates will drop out after each of those states, consolidating support behind other candidates.

On December 10, 2008, Rudy Giuliani led in national polls by 5 points. But he was weak in Iowa. He lost Iowa badly, and then faded to the background.

Hillary Clinton also led the national polls that day, but Obama had eked into the lead in Iowa surveys. Many Democratic voters loved Obama but didn't think a black man could get elected. When Obama won Iowa, that fear vanished. Obama immediately rose in the polls in every state.

So, lesson No. 1 is this: national polls, where Trump's lead is the biggest and the most consistent, are nearly meaningless. Take any December 10 before the presidential primaries in this millennium, and the national polling leader that day has not won Iowa or the nomination in any contested primary.

Here's another telling fact: the more that polls refine for the likelihood of a respondent actually voting, the worse Trump does.

Trump leads Rubio in New Hampshire, 21 percent to 17 percent in a recent Adrian Gray Consulting poll. But when the pollster screened to "Likely NH GOP Voters," the result was a tie—19 percent Rubio to 18 percent Trump. Refining further to "Very Likely NH GOP Voters," Rubio takes a nominal lead within the margin of error, 19 percent to 17 percent.

Iowa's latest poll tells a similar story. Ted Cruz leads Trump in Monmouth's poll, 24 percent to 19 percent. The pollster notes, however, that if they limited the poll to voters who had participated in caucuses in the past, Cruz would lead Trump 25 to 16, nearly doubling his lead.

This pattern suggests how things might shake out in the coming weeks. As pollsters sharpen their likely-voter screens, Trump's numbers will drop.

Trump's numbers will drop still further as voters get more serious about choosing a nominee. A majority of Republicans in both Iowa and New Hampshire are still undecided. Every Trump supporter I spoke to in Iowa immediately named a secondary candidate, Paul, Rubio, or Cruz.

Americans hate Congress, Politicians, Washington, and the Republican Party. Telling a pollster you will vote for Trump may be more of an expression of disdain for the "Establishment" than an actual expression of voter intention.

Put another way: Mad-as-Hell voters have dated Trump, but they're ready to marry Cruz or Rubio.

But here's what will not happen: Trump will not crash and burn because he says some terrible thing. Attacking McCain for being a POW, making misogynist comments against Fox News hosts, mocking reporters for being disabled, and proposing a religious test for immigrants: these comments all help Trump. They help Trump because they evoke freak-outs from liberals and establishment types, which a certain kind of voter loves more than anything.

In today's world of political correctness, everything evokes freakouts. It's sexist to criticize Fed Chair Janet Yellen. It's racist to call Obama skinny. Even defending the freedom of speech is oppression. Some Americans, sick of this idiotic speech policing, have developed a habit of gravitating towards the offensive.

So Trump isn't going to blow himself up. He's simply going to fade away.

Timothy P. Carney, The Washington Examiner's senior political columnist, can be contacted at tcarney@washingtonexaminer.com. His column appears Tuesday and Thursday nights on washingtonexaminer.com.