Donald J. 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 Mr. 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 Mr. 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 Mr. Trump, have remained on the sidelines.