It’s hard to recognize the Democratic Party these days. In recent decades, it’s been a divided, brawling tribe. But this year, Democrats are one big, happy family.

Sure, there was grumbling from the left over President Obama’s agreement to keep tax cuts in place for couples making between $250,000 and $450,000 a year. But that quickly gave way to satisfaction that Obama had won the “fiscal cliff” fight, and growing confidence that he can win the next round over the federal debt ceiling as well.

Two factors have given Democrats this unusual sense of unity and well-being: the surprisingly big margin by which Obama won November’s presidential election, and the obduracy of House Republicans in refusing to notice that their arguments lost. If the GOP wants to keep alienating voters by telling them they’re wrong, the Democrats are happy to offer them more opportunities.

In recent weeks, the president has dared Republicans to refuse to raise the federal government’s debt ceiling. He has made clear that he will push for significant immigration reform and gun control measures, issues on which Democrats believe the GOP is out of step with mainstream opinion.

“The dominant sense among Democrats is that the party is on a roll,” said William A. Galston, a former Bill Clinton aide now at the Brookings Institution. “And why not? 2012 was a victory, after all.”

But there’s a cloud on the Democrats’ blue horizon: the unsolved problems of federal spending and the national debt.

As Obama has acknowledged, if federal spending continues to grow unabated — especially on the big entitlement programs of Medicare, Medicaid and Social Security — there won’t be enough money in the Treasury to pay for much else. The modest increases in tax revenue he just obtained won’t be significant over the long run.

Even if the president wins the coming fight over the debt ceiling, there’s no escape from the spending problem. By the end of February, Congress must either undo the automatic sequester of federal funding that it ordained in earlier bouts of budget wrestling or see savage cuts to domestic and defense programs. And by the end of March, the government needs a new spending bill to allow agencies to keep operating.

That’s when the Republicans’ best chance to press for spending cuts will come. And that’s when Democrats could revert to their traditional feuding ways — because, as even Obama agrees, the biggest target for reductions is one of the Democrats’ favorite programs: Medicare.

“I agree with Democrats and Republicans that the aging population and the rising cost of health care (make) Medicare the biggest contributor to our deficit,” Obama said during the fiscal cliff negotiations. “I believe we’ve got to find ways to reform that program without hurting seniors who count on it to survive.”

Medicare already accounts for about 15 percent of federal spending (not counting interest), and the Congressional Budget Office projects that the cost will nearly double in 10 years if no changes are made. Bringing federal spending under control without touching Medicare simply isn’t practical.

But it’s a prospect that chills many Democrats because defending Medicare and Social Security benefits is the clearest unifying doctrine their party has, just as resisting tax increases is for Republicans.