To Donald Trump's boosters, the theory for how he could win the presidency hinged on the idea that he'd build on Mitt Romney's performance among white voters, putting even blue states like Pennsylvania in play. But things aren't quite working out that way.

A new poll from Quinnipiac University (which has tended to be favorable to Trump) finds that he is underperforming Romney among whites in the key swing states of Pennsylvania, Ohio, and Florida — all three states that Romney lost.

The erosion of support among white voters helps explain why Trump has lagged in the states. In the same poll taken last month, Trump was tied or ahead in all of the states, now he's taken a hit in Ohio and Florida (which are still close), and has cratered in Pennsylvania, where he's down 10 points to Hillary Clinton.

Pennsylvania is illustrative. Last month's poll, which had Trump up 2 points in the state, he was winning white voters by 18 points. In the poll released on Tuesday, he was only up 3 points among white voters, and saw Clinton surge to a 10-point lead in the state.

No candidate since 1960 has won the presidency without carrying at least two of these three states, and in Trump's case, it's hard to see any path to the White House that doesn't involve a clean sweep of all of them.