WASHINGTON -- Here's why getting to a deal on the debt ceiling is so complicated.

President Obama's main goal is to get through this fight with the government still running and his support from the political center intact, even if this means substantial concessions to Republicans.

House Republican leaders want to get by without inciting a revolt among right-wing tea partiers, which means they're having trouble accepting Obama's concessions.

And the Senate -- well, the Senate resembles the Balkans without a peacekeeping force. Poor Harry Reid. The Democratic leader's caucus sprawls from ardent progressives to moderates from conservative states absolutely petrified of casting votes that might endanger their seats in 2012. Senate Republican leader Mitch McConnell senses that a GOP majority is just around the corner, and wouldn't mind dragging this struggle out.

The House Democrats would seem to be fodder in all this dysfunction. Everyone knows that Speaker John Boehner will need Democratic votes to get a deal passed because some of his conservatives will vote against anything that doesn't reduce our federal establishment to the size of the government of Belize. The most progressive half of the House Democratic caucus will likely oppose the final deal as a sellout.

That could give the middle-of-the-road half of the House Democratic contingent the swing votes. Paradoxically, this creates an opportunity for those holding the weakest hand at the table. Democratic leaders Nancy Pelosi and Steny Hoyer have a chance to (1) stiffen Obama's spine, at least a bit; and (2) tell Boehner that if he wants Democratic votes, he'll have to concede something. He can't keep the tea party at bay and have a deal moderate enough to win Democrats at the same time.

Although Obama spoke Tuesday of reaching a "big" deal on the deficit, he's not looking to solve the whole problem before the debt-ceiling deadline of Aug. 2. What he's looking for is somewhere around $2.5 trillion in reductions over a decade. This would allow Congress to pass a large enough debt-ceiling increase to get us through the next election. That's when the voters will be asked to settle the fundamental issues, especially whether we need to raise taxes on the wealthy.

Getting $1.5 trillion of those cuts is not exactly easy, but it's doable, given that there are reductions on which Obama and the Republicans agree. The math also helps: $1.2 trillion in cuts saves you another $300 billion in interest costs. The rest is harder, especially on two sets of issues: any cuts in Medicare and Medicaid, even if they are aimed at providers and not beneficiaries; and all attempts to increase revenues -- a dreaded word conservative Republicans will always translate into "taxes."