Barack Obama and John McCain are slowly beginning to get it: For the next President, this week’s financial market meltdown has changed everything.

Suddenly, their grandiose promises of new tax cuts and ambitious spending are sounding more hollow than ever. An $11.3 trillion national debt will do that to you every time.

$11.3 trillion is the new cap on the public debt requested by the Bush Administration in the wake of its proposed financial market bailout. That assumes we will end up spending $700 billion of taxpayer’s money hauling these piles of financial garbage off to the dump.

As most TaxVox readers know by now, the TPC estimates that, including interest costs, Obama’s tax plan would raise the debt by an additional $3.5 trillion over the next decade. McCain would add more than $5 trillion.

This numbers are mind-numbing. With luck, the cost of the bailout will be transitory. But it is likely to still cost hundreds of billions, and will suck up massive amounts of the new Administration’s time and political capital. Grudgingly, both the candidates and their surrogates are starting to acknowledge they may have to adjust their own priorities.

This afternoon, I moderated a debate between Doug Holtz-Eakin, McCain’s senior policy advisor, and Jared Bernstein, an informal advisor to Obama. At that event, sponsored by the National Economist’s Club, Holtz-Eakin conceded that “it has certainly gotten a lot harder” for McCain to achieve his promise of balancing the budget in four years, although he insisted the Republican would still try. Bernstein came close to admitting that balance any time soon was impossible. Keeping deficits below 3% of GDP seems to be the campaign’s new goal. At the same time, Obama himself was admitting that some of his spending plans, including health reform, may need to be scaled back or delayed.

More than most campaigns, this one increasingly has had an air of unreality about it. We are all used to candidates making promises they cannot keep. But Obama and McCain had taken that game to a new level. Now, both may be realizing that all of their grand plans will end up on the back burner. Whoever is elected will spend his first year in office dealing with only two issues: the financial markets and the economy at home, and extricating the U.S. from Iraq.

All the rest: climate change, entitlement reform, health care, tax reform, will never escape the in-box.