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.