How much savings is plausible on the spending side? Medicare, Medicaid and Social Security will account for almost half of spending by 2015. Even if we reform Social Security, which we should, any plausible plan would phase in benefit changes to avoid harming current beneficiaries  and so would generate little savings over the next five years. The health reform act included substantial savings in Medicare and Medicaid, so there aren’t further big reductions available there in our time frame.

The other half of the budget is mostly net interest (which is not negotiable unless we renege on our debt) and discretionary spending. Discretionary spending is split roughly equally between defense and non-defense spending. The defense component already assumes a phase-down in both Iraq and Afghanistan; saving an additional 5 percent of the Pentagon’s base budget would be a substantial accomplishment and would yield about 0.2 percent of G.D.P. Cutting 5 percent out of non-defense discretionary spending, a stretch politically, would save about as much.

It would be tough, then, to squeeze more than a half percent of G.D.P. from spending by 2015. Additional revenue  in the range of 0.5 to 1.5 percent of the economy  will therefore be necessary to reduce the deficit to sustainable levels.

How would we do this?

One possibility would be to establish a new source of revenue, perhaps through revenue-increasing tax reform, and possibly including a modest value-added tax (that is, a V.A.T. of 5 percent to 6 percent). This approach has many potential benefits, including the opportunity to improve our tax code by cutting back on loopholes and shifting toward a consumption-based tax system. It is also politically impossible, at least in the era of the 60-vote Senate. Those who fear a V.A.T. have little reason to worry  the votes aren’t there.

The beauty of extending the tax cuts for only two years is that canceling them doesn’t require an affirmative vote. It happens by default, so Congressional deadlock works in its favor. And it would essentially solve our medium-term deficit problem, reducing the deficit by $200 billion to $350 billion a year from 2015 to 2020.

Like all plans, this one isn’t perfect. Some may complain that higher marginal tax rates, even if deferred until 2013, will cripple small businesses and economic activity. It’s hard to believe, however, that effectively returning the tax code to its 1990s form would lead to economic catastrophe, especially when many leading Republican economists  including Alan Greenspan and Martin Feldstein  agree that we can’t afford to continue the tax cuts forever. More troubling, middle-class and lower-class families would be saddled with higher taxes. That’s a legitimate concern, but also a largely unavoidable one if we are to tackle the medium-term fiscal problem.

Finally, a key part of this deal is actually ending the tax cuts in 2013  and that will surely require a presidential veto on any bills to extend them after that. (Failing to follow through would be particularly problematic if the high-income tax cuts are made permanent  at a 10-year cost of more than $700 billion.) Minimizing this risk requires as much upfront clarity and commitment as possible, including a strong and unambiguous veto threat from the president.

Senate Democrats and Republicans almost never come together anymore. This month, they should fight the dual deficits rather than each other. Let’s continue the tax cuts for two years but end them for good in 2013.