Trump Debate Advice: No More Mr. Nice Guy, Mitt

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Businessman Donald Trump shakes hands with Republican presidential hopeful Mitt Romney after announcing his endorsement of Romney at Trump International Hotel & Tower on February 2, 2012 in Las Vegas.

The Donald's advice to Mitt Romney for Wednesday's debate against President Barack Obama: No more Mr. Nice Guy.

“I think the nice guy stuff is over,” Donald Trump told CNBC’s “Squawk Box” on Tuesday.

“I think it’s going to be a much different Mitt Romney,” the real estate magnate said.

“He’s going to be very strong and very smart, and he’s going to attack the president on what he can attack him on, which is the economy and jobs.”

Trump, who has been a vociferous critic of Obama, said he canceled a business trip to Europe in order to watch the debate. (Read More:Trump Now Taking Aim at Obama's Collegiate Background.)

Romney is running behind in the polls, triggering a flood of hand-wringing among Republicans about his campaign. (Read More: Kudlow: Have Romney’s Closest Advisors Failed Him?)

Trump, however, said there's still time for the GOP nominee to shift his momentum.

The remaining 35 days to the general election “is an eternity in politics … so many things can happen for better or for worse,” said Trump.

“Tomorrow is Day One. The debates are going to be very, very important,” he said.