This analysis of Mitt Romney's budget promises was originally posted on August 14. But with Romney set to officially accept the GOP nomination for president tonight, it's worth taking another close look at the policy proposal he's placed at the center of his campaign.

The Republican budget might keep President Obama’s cuts to Medicare. But a Romney administration wouldn’t.

Lanhee Chen, the campaign’s policy director, left no room for doubt in his statement: “A Romney-Ryan Administration will restore the funding to Medicare, ensure that no changes are made to the program for those 55 or older, and implement the reforms that they have proposed to strengthen it for future generations.”

Avik Roy, a health-care policy adviser to Romney, doubles down. “Whatever you think of Obamacare’s cuts to Medicare, the fact is that a Romney administration would repeal them,” he writes.

But then how will a Romney administration make its budget math add up?

Romney is not going to make these kinds of cuts to the federal pie. (Shannon Stapleton — Reuters)

Consider what Romney has promised. By 2016, he says federal spending will be below 20 percent of GDP, and at least 4 percent of that will be defense spending. At that point, he will cap federal spending at 20 percent of GDP, meaning it can never rise above that level.

All that’s hard enough. Romney will have to cut federal spending by between $6 and $7 trillion over the next decade to hit those targets. As my colleague Suzy Khimm has detailed, those budget promises already require cuts far in excess of what even Paul Ryan’s budget proposes.

But Ryan’s budget includes more than $700 billion in Medicare cuts over the next decade, Romney’s budget won’t. And Romney promises that there will be no other changes to Social Security or Medicare for those over 55, which means neither program can be cut for the next 10 years. But once you add up Medicare, Social Security and defense and you’ve got more than half of the federal budget. So Romney is going to make the largest spending cuts in history while protecting or increasing spending on more than half of the budget.

The Center on Budget and Policy Priorities indulged this idea back in May. If Social Security and Medicare are spared from cuts, then to get federal spending under 20 percent of GDP while holding defense spending at 4 percent of GDP, “all other programs — including Medicaid, veterans’ benefits, education, environmental protection, transportation, and SSI — would have to be cut by an average of 40 percent in 2016 and 57 percent in 2022.”

Consider what the Romney campaign, then, is saying: If Romney is elected, then by his third year in office, every single federal program that is not Medicare, Social Security, or defense, will be cut, on average, by 40 percent. That means Medicaid, infrastructure, education, food safety, road safety, the postal service, basic research, foreign aid, housing subsidies, food stamps, the Census, Pell grants, the Patent and Trademark Office, the FDA — all of it has to be cut by, on average, 40 percent. If Romney tried to protect any particular priority, it would mean all the others have to be cut by more than 40 percent.

That’s not even remotely plausible. The consequences would be catastrophic. The outcry would be deafening. And Romney has shown no stomach for selling such severe cuts.

Consider that, even as we speak, Romney is running away from the unpopular bits of the Ryan budget, which delivers far less devastating cuts than what Romney is promising. Does anyone really believe that he will take office and cut education by 40 percent? That he will take office and, after running away from specifics during the campaign, propose what would surely be the most unpopular budget in American history?

And does anyone believe that the real Romney is the guy who made these outlandish budget promises in order to win a Republican primary, rather than the guy who is disavowing Ryan’s Medicare cuts mere days after naming him to the ticket?

This is simply not a credible budget plan, and Romney’s fast retreat from Ryan’s most unpopular cuts makes it even less credible. And yet Romney, who has never released the specific cuts that would make his numbers add up, repeatedly touts it on the campaign trail, and the media dutifully reports his promises to cut federal spending by more than $500 billion in 2016, and in fact to balance the budget by the end of his second term, which would require far larger cuts than what I’ve outlined here, despite the fact that everyone basically knows these cuts aren’t credible and will never happen.

I’m not sure what alternative there is, exactly, except to say, as clearly as possible, Romney’s budget plan is a fantasy, and it will never happen. I would revise that opinion if Romney released a list of specific cuts that achieved his spending goals and showed himself willing and able to take the heat for them. But I don't think that's going to happen.