If Donald Trump once got away with making impossible promises for achieving federal savings, he isn't anymore.

The bipartisan Committee for a Responsible Federal Budget blasted the billionaire GOP presidential front-runner on Friday for proposing "mathematically impossible" plans involving tax cuts and Medicare drugs, and praised Fox News debate moderators for calling Trump out on his proposals at Thursday night's debate.

"The moderators of last night's Fox News Republican debate deserve praise for challenging Donald Trump on his unrealistic and mathematically impossible plan to pay for his budget-busting tax cut," said the group's president, Maya MacGuineas.

According to an analysis by CRFB, Trump's major tax initiatives would add $12 to $15 trillion to the debt. And the real estate mogul has claimed he would save $300 billion a year in savings by allowing Medicare to start negotiating drug prices with pharmaceutical companies, even though Medicare spends only a third of that amount on prescription drugs every year.

Fox News moderator Chris Wallace probed Trump on Thursday night to explain how he is promising to save more prescription drug spending every year than even exists.

"You say that Medicare could save $300 billion a year negotiating lower drug prices," Wallace said. "But Medicare total only spends $78 billion a year on drugs."

"I'm not only talking about drugs, I'm talking about other things," Trump responded. "We will save $300 billion a year if we properly negotiate. We don't do that. We don't negotiate. We don't negotiate anything."