In its most recent 30-year plan, the Navy set its goal of maintaining 310 to 316 total vessels by building around eight new ships a year. But in a report this month, the Congressional Research Service noted that the Navy's own projections showed it would miss that bar for all 30 years, "and experience shortfalls at various points in ballistic missile submarines, cruisers-destroyers, attack submarines, and amphibious ships."

Even making those concessions, the Congressional Budget Office has suggested that the Navy's plans would extremely expensive. Since 1983, we've spent an average of almost $15 billion per year building destroyers, subs, aircraft carriers and such. The CBO believes the current 30 year plan would require an annual $20 billion, on average.

Instead of building eight ships a year, Romney has suggested we crank out 15. As Wired's Spencer Ackerman has reported, this would likely cost an additional $35 to $40 billion over half a decade, depending on which ships his administration actually chose to produce.

And again, the investment might be worth it. Spending more money to build advanced warships would a decent jolt of stimulus. It would help reinforce our naval dominance at a time when China is trying to project itself as a maritime power. The problem is, nowhere does Romney suggest how he would pay for it. He's proposed spending an additional $2.1 trillion on the military over 10 years. He's also suggesting massive tax rate cuts (with vague but promised offsets) and steep budget cuts that somehow spare Medicare and Social Security. There's no giant couch on Capitol Hill where the spare change he'd need to make this happen is hiding.

It wasn't absurd or backward-looking for Romney to suggest we might need to invest in our navy. But as Obama noted, we have to work within the world of fiscal reality. "That's not reflected in the kind of budget you're putting forward, because it just doesn't work," he told Romney, in what was probably the heart of their exchange. The bayonets line was funny. But the idea that we can have all the guns, butter, and tax cuts we want is the real joke.