A month ago Barack Obama was defeated and diminished, his opponents empowered, his supporters so dispirited that they looked upon their onetime hero the way John Greenleaf Whittier regarded Daniel Webster after the Compromise of 1850: “So fallen! So lost!”

That was before Obama grabbed his protractor and worked the lame-duck angles, negotiating a tax agreement, prevailing in the battle over gays in the military and gaining approval of an arms-control treaty. He who had fallen had now risen; he who had lost had now won. Even so, there remained liberals who — to push the 1850 comparison — found themselves embracing the political position of their nemesis, John C. Calhoun: “I hold concession or compromise to be fatal.”

Shifting politics

Now the geometry of Washington has shifted again — get your pointy compasses out — and Obama looks up Pennsylvania Avenue to see a Congress and his prospects transformed. The post-election period the president described as “the most productive post-election period we’ve had in decades” has vanished along with the strategies that produced it. Time to recalibrate.

All of this is happening at a critical time for Obama, for the Republicans, for the country.

In the first decade of this century, American domestic politics resembled A.J.P. Taylor’s classic definition of German history: “a series of turning points where nothing turns.” But now, with monster deficits and massive demographic changes afoot, the country cannot afford to continue on a straight line — a line that would lead to financial insolvency and generational warfare.

The first baby boomers reached age 65 a few days ago. Millions will follow, as will a gigantic case of entitlement enmity if the deficit, Social Security and Medicare aren’t addressed, not next year, not the year after next, but this year — before Obama’s rivals start announcing their presidential plans, before the White House goes on a campaign footing, before the country’s attention is diverted to the drama inherent in the caucuses and primaries.

Budget plan is critical

This is why the president’s budget, which is to be announced only four weeks from now, is of vital importance. He almost certainly will incorporate some of the elements of the bipartisan commission that recommended a tax overhaul and spending cuts. The House Republicans will embrace the latter, the Senate Democrats the former, but in truth the country needs both.

The movement of the baby boomers into retirement age — boosting enrollment in Social Security over the next 20 years to 73 million people, up from 47 million now — is only one of two vital demographic factors that have immense political implications.

The other is the continued movement of population, and thus of political power, from the North and East to the South and West. This migration seeps congressional representation — and Electoral College votes — from states congenial to Obama and the Democrats to states that view them with skepticism if not outright suspicion, and its effects will be dramatic in the 2012 elections. Demography is destiny, both in financial ledgers and on political maps.

For those reasons alone, the politics of 2011 are unusually complicated. Now add a third factor, a peculiarity of the calendar.

Next year, when a third of Senate seats are up for grabs, the Democrats will find themselves disproportionately at risk. Some 23 of the 33 seats being contested in 2012 now are occupied by Democrats. Incumbents in such races ordinarily have the advantage, of course, but this is a period of raging resentment toward the political class, and it is always better for a party to have its rivals’ seats at risk rather than its own.

The president has said that the last few weeks of 2010 showed “that we’re not doomed to endless gridlock,” but maneuvering in an environment of divided government and high deficits is hard, as Bill Clinton, who didn’t have the deficit burden that weighs down Obama, can testify.

With its dinners with civil rights groups and union officials and its overtures to interest groups, the administration is mending fences with its base. With its newfound power and chairmanships, the Republican congressional leadership is feeling its oats.

Unified action unlikely

Coming together, these two forces — one full of suspicions, the other threatening subpoenas — show signs of prying the parties apart.

House Republicans are determined to make a big gesture this month — to vote to repeal Obamacare even though they know the Democrat-controlled Senate will not go along and even though their new ceremonial practice of reading the Constitution out loud should remind them that Article I, Section 7 gives the president a veto over congressional legislation.

Once January passes and Congress settles down, however, the president and his Republican rivals have some decisions to make. Many of them come under this rubric: Are we in office to make statements or to make laws — and progress?

This is not a question addressed only to Republicans, who made hay in 2009 and 2010 by making obstruction their tool of choice. It is also a question for Democrats, who in the majority often didn’t give the Republicans a fighting chance, only to see the Republicans fight back and gain the main chance.

Some Democrats still smart from the president’s compromise on the Bush tax-cut extensions. But they forget for whom they voted in 2008.

Obama indicated during his campaign that he was more interested in competence than ideology. The Democrats (who thought they had nominated a third Roosevelt, or maybe a third Johnson) didn’t believe him, and the Republicans (who, just as erroneously, enjoy taunting him as a socialist) didn’t either, and so when he broke his allies’ hearts in December by breaking bread with the Republicans, Democrats felt betrayed rather than redeemed, and Republicans felt befuddled rather than bewitched.

If you buy that the December compromise was the product of the Real Obama (and not one shamed by his shellacking) colluding with the Real Republicans (and not those playing to the scorched-earth chorus), then it might be possible to see a way forward for Washington and its reconstituted Congress. It takes two to make a deal, or to make a difference. It takes only one to make a real mess of things.