A general election matchup between Donald J. Trump and Hillary Clinton became all but certain on Tuesday after Mr. Trump’s decisive victory in Indiana.

He would begin that matchup at a significant disadvantage.

Yes, it’s still a long way until Election Day. And Mr. Trump has already upended the conventional wisdom many times. But this is when early horse-race polls start to give a rough sense of the November election, and Mr. Trump trails Mrs. Clinton by around 10 percentage points in early general election surveys, both nationally and in key battleground states.

He even trails in some polls of several states where Mitt Romney won in 2012, like North Carolina, Arizona, Missouri and Utah.

Could Mr. Trump overtake Mrs. Clinton? Sure. Mrs. Clinton is very unpopular herself. Her polling lead is a snapshot in time, before the barrage of attack ads that are sure to come her way. There have been 10-point shifts over the general election season before, even if it’s uncommon. But there isn’t much of a precedent for huge swings in races with candidates as well known as Mr. Trump and Mrs. Clinton. A majority of Americans may not like her, but they say they’re scared of him. To have a chance, he’ll need to change that.