When it comes to Donald Trump, two strands of thought appear to be strengthening simultaneously in different camps of the Republican Party: He can’t possibly end up as the presidential nominee, and it looks increasingly likely that he may do just that.

On the first front, a series of prominent GOP pundits and strategists in recent days have issued barbed denunciations of the real-estate mogul. Mr. Trump as nominee “would pose a profound threat to the Republican Party,” wrote Peter Wehner, a former adviser to President George W. Bush. A Trump win, said former Bush speech writer Michael Gerson, would mark “a massive ideological and moral revision” of the GOP. Mark Salter, a longtime campaign adviser to Sen. John McCain, hit a more profane note, calling Mr. Trump “a mean-spirited, lying jerk.”

At the same time, if you trust the polls, Mr. Trump is gaining strength in the early states, and lags behind only in Iowa, where voters are set to kick things off in all of 11 days. Since 2000, among GOP candidates, the size and durability of Mr. Trump’s lead in national polls has been topped only by George W. Bush, who went into the Iowa caucuses with the support, on average, of around 60% of likely GOP voters. Mr. Trump now stands at around 35%, according to the Real Clear Politics running tally. His nearest rival, Texas Sen. Ted Cruz, comes in at 19%.

Mr. Trump has his vulnerabilities. A solid third of Republicans say they can’t see themselves supporting him as the nominee, according to the latest Wall Street Journal/NBC News poll. (Then again, that is the lowest number of the cycle, and down from 74% last March.) In head-to-head races, according to the same poll, he would lose to Mr. Cruz in the primaries and to both Hillary Clinton and Sen. Bernie Sanders in the general election. In a hypothetical matchup against Mrs. Clinton, just 19% of nonwhite voters support him.

On the other hand, Mr. Trump has shown remarkable staying power, and appears to be assembling a disparate block of supporters that could portend surprises in many states. He continues to draw huge crowds in unlikely places, like Lowell, Mass., this month. In WSJ polling, his support among primary voters is similar to what then-Sen. Barack Obama had at this point in 2008, and stronger than Sen. John McCain that year or Mitt Romney four years later.