Treasury Secretary Steven Mnuchin. Mark Wilson/Getty Images Treasury Secretary Steven Mnuchin says the Trump administration hopes that the new tax plan rolled out Wednesday brings down taxes for the middle class, but it isn't a sure thing.

In an interview with George Stephanopoulos on "Good Morning America" on Thursday, Mnuchin was asked if he could guarantee that no one in the middle class would pay more under President Donald Trump's plan.

"That's our objective, absolutely," Mnuchin said. When pressed by Stephanopoulos if it was a guarantee, Mnuchin demurred.

"I can't make any guarantees until this thing is done and on the president's desk," Mnuchin said.

Given the long legislative process, pushback from Democrats, and likely attacks from deficit hawks in the Republican Party, there is a good chance that the tax plan that makes it through Congress will look different from the one-page outline released by the White House on Wednesday.

Given these factors, it's fair for Mnuchin to hedge his bets a little, but the lack of details in the outline — including the level of income associated with each of the three proposed tax brackets — leaves a lot of unanswered questions about how the plan would affect the middle class.

Mnuchin also would not promise that the plan wouldn't result in an absolute tax cut for wealthy Americans, which the Treasury secretary said in February wouldn't happen under Trump's tax plan. Democrats even called this promise the "Mnuchin rule."

Mnuchin rebuffed the suggestion of Trump releasing his tax returns so Americans could see how the new plan would affect the president's taxes.

"This isn't about the president's taxes," Mnuchin said. "This is about what's good for the American people."

Watch part of Mnuchin's interview with "Good Morning America":