Many political pundits have tried to explain the Trump "phenomenon", i.e., his staying power atop the GOP polls despite statements that would sink many another candidate.
The explanation is really not that difficult. Donald Trump has cornered the George Wallace vote. In most years that would not seem like much, but in this unusual election year the Wallace vote has taken on a disproportionate impact in the GOP primaries.
Few will recall that in 1968 George Wallace ran for president on an avowedly racist, segregationist platform. It was also a populist platform economically. His appeal was largely limited to lower class whites of more limited education, but he nonetheless received nearly 14% of the vote nationwide, and carried five southern states.
Back in the 1960's and 1970's, many Wallace type voters identified themselves as Democrats, for historical reasons. They tended to either stay out of national elections, or vote Republican, particularly if someone like a Goldwater was running. Many more were independents. These days, the parties have re-aligned, and Wallace type of voters are more firmly in the GOP camp (although some are still independents) to both the benefit and detriment of the Republican Party.
Notwithstanding that realignment, Wallace voters have had a fairly limited impact on Republican presidential nominating contests. In a typical election year, with maybe three serious GOP candidates pursuing the presidential nomination, the Wallace type vote would not be so evident. A leading "centrist" candidate, such as a Romney or McCain, would have roughly 60% of the vote in polls in such a contest, and a couple other more right wing candidates might carve up the Wallace vote, maybe one getting 16% and the other getting 10%. In such a contest, the Wallace voters are out there, but they in no way dominate, or even significantly affect, the course of the race.
This year is unusual. There are more than a dozen candidates, with no heir apparent. Trump went right out of the gate for the Wallace type voters, proclaiming in his first speech that Mexico was sending rapists and murderers across the border. He has continued to make a series of bigoted and racist comments since then, and he has stayed at a pretty steady 20-25% in national polls of Republicans. (In head to head polls against Democrats Hillary Clinton and Bernie Sanders, however, Trump typically polls the worst of the leading Republicans.)
But remember, Republicans make up roughly one third of the electorate. So 25% of that one third is less than ten percent of the overall electorate--basically, the Wallace vote. These voters have always been around. They probably always will be (European democracies also have their hard core right wing nativist political groups, who usually get around 10% of the vote, barring something unusual.)
Trump is the first serious candidate for President since Wallace to overtly court bigoted voters. Unlike Wallace, whose credentials as a racist were quite well established before he ran for President, Trump showed few signs of outright bigotry before running for office (he certainly looked like a run of the mill sexist, but not a bigot). Trump is no dummy--we suspect that he made a very calculated move to corral the nativist right, and has been fighting to hold on to it ever since. Of course, that makes him practically unelectable in a general election, but maybe he figured that would be a bridge to cross later.
In any event, Trump's "enduring" appeal is not that difficult to explain. Anyone can put together the hard core Wallace vote if they are willing to be overtly racist and bigoted. Few have tried, because it is not typically a winning strategy in the long run. But don't expect Trump's support to erode or fade--as long as he keeps up what others view as outrageous statements, he will maintain the Wallace vote--they have nowhere else to go.