"It is catastrophic what he's done," Trump said during an interview with CNBC. "If they lose, it will be the single biggest reason why the Republicans lost: the Ryan plan."
Trump went on to warn Romney against embracing the plan too strongly, indicating it could come back to hurt his general-election bid.
"I think the worst thing he can do is strongly embrace that budget if he wants to get elected," Trump said. "This will be the single worst move in the Republican Party for many years. This is going to be catastrophic."
That horse might already be out of the stable. Romney has repeatedly praised the Ryan plan, and campaigned heavily with the congressman before the Wisconsin primary. Their relationship seemed tight-knit enough that Ryan fielded questions about possibly joining the Republican ticket as Romney's running mate on "The Today Show" Tuesday morning.
In a statement following the release of the budget proposal, Romney praised Ryan “for taking a bold step toward putting our nation back on the track to fiscal sanity.”
“As president, I look forward to working with Chairman Ryan and his House Republican colleagues to pass bold reforms that restore America’s promise,” Romney continued.
Trump has a history of disparaging Ryan's proposals. Last month, he tweeted that the plan was "very dangerous for the Republicans," while labeling a similar proposal in 2011 a "death wish" for Republicans.
On Tuesday, the reality television star questioned Ryan's political sense.
"Perhaps Paul Ryan is a bright guy but he is not a good chess player," Trump said. "This gentleman is not a good chess player, that's all I can tell you."