Donald Trump has snatched three straight victories, built a powerful lead in national polls and knocked several opponents out of the 2016 presidential race. His path to the Republican nomination looks wider than ever. But it could still contain pitfalls and roadblocks. Here are some of the ways he could still stumble.
Billionaires get into the game
Top Republican donors have shied away from confronting Trump, but at some point the party's bankrollers may get serious about saving it from a man they view as a catastrophe. If they did, this could represent a serious threat to Trump. Imagine tens of millions of dollars in attack ads blanketing the landscape of primary states.
But while billionaires like Robert Mercer and Paul Singer have given generously to candidates, only one superwealthy clan, the Ricketts family, has given a few million dollars to an anti-Trump group. Even the conservative Koch brothers, who are seen as deeply resistant to Trump, have remained on the sidelines.
An alternative emerges
In a race that began with 17 candidates, Trump has benefited from deftly playing off his opponents against one another. With four left, he can still control the race with far less than a majority of the vote.
Neither Sen. Marco Rubio of Florida nor Sen. Ted Cruz of Texas is likely to give way willingly, or join forces with the other. And two long-shot candidates would still snarl efforts to unite the anti-Trump vote: Gov. John Kasich of Ohio and Ben Carson have plodded on despite their slim chances. But should the field suddenly dwindle to one or two rivals of Trump, perhaps after Super Tuesday, it would test the breadth of his support as never before.
Debates turn disastrous
Trump has staggered through the debates by sheer force of personality, at times inflicting real damage to himself in the process. In South Carolina, late-deciding voters rejected him after his volcanic attacks on former President George W. Bush in the final debate.
The stage may get even tougher for him starting Thursday night in Houston: He can no longer count on a large field of opponents to shield him from a formidable puncher like Cruz, or from Rubio, who is also seeking to break through.
A big-state stumble
Trump has a lead in the delegate count, but ultimately he will need a majority to lock up the nomination before the convention. He now has 81 delegates, against fewer than 20 for each of his opponents. He needs 1,237 to clinch the nomination.
If Trump continues to build momentum, he could approach that number with startling speed as states with winner-take-all delegate rules vote later in March. Yet his opponents also have a window to catch up with him, or at least to deny him the majority he needs, by making a stand in big states like Florida and Ohio, where Trump victories might make him truly inevitable.
His own worst enemy
Republicans have bet for months that Trump would destroy his own campaign through sheer intemperance or incompetence. So far, they have been disappointed.
He has grievously offended Republican leaders and conservative voters, attacking party heroes like Bush and Sen. John McCain and defending hated adversaries like Planned Parenthood. He has been stingy, declining to run the tens of millions of dollars in ads typically required to win presidential elections.
But just because Trump has not yet paid a price for his lack of discipline does not mean he never will. A monumental blunder could be much costlier now than it would have been earlier in the race. For many Republican leaders, the question is whether Trump's eventual self-immolation comes before he wins the nomination, or in November.