Congress returns from Thanksgiving with an overflowing agenda and little time to deal with some urgent items. The federal government will partially shut down if lawmakers fail to pass a funding bill by midnight Friday. The Treasury Department has warned that the United States might default if Congress does not raise the debt limit by Dec. 15. And Senate Democrats aim to advance a sprawling social spending and climate bill, representing the core of President Biden’s domestic agenda, by Christmas.

Though the deadline to avoid a shutdown will arrive first, the biggest threat, by far, is that Congress might fail to raise the debt ceiling by mid-December, leading to a catastrophic federal default that throws the world economy into an immediate crisis. Earlier this fall, Democrats and Republicans played a risky and destabilizing game of chicken over the debt limit, with Senate Minority Leader Mitch McConnell (R-Ky.) withholding GOP help and demanding that the majority use the complex and time-consuming “reconciliation” process to hike the limit with only Democratic votes. Mr. McConnell finally relented in October, but he did so only after the country got dangerously close to default, and he allowed only a brief reprieve, which is now almost up.

The good news is that Mr. McConnell and Senate Majority Leader Charles E. Schumer (D-N.Y.) have been discussing the looming debt limit deadline, hinting that this round might be less chaotic. The bad news is that former president Donald Trump and his acolytes have attacked those Republicans who helped the Democrats avoid default in October, a sign of the political risk Senate Republicans might face for doing the right thing a second time. Some Senate Republicans appear determined to force Democrats to go through reconciliation this time; many Senate Democrats seem equally determined to avoid that route.

The national interest requires that Congress leave no doubt that the United States will meet its obligations. When it comes time to raise the limit to finance the spending lawmakers have already approved, the measure should fly through Congress by acclamation. Instead, the debt limit has become the locus for increasingly alarming rounds of political gamesmanship, with stakes that are almost indescribably high. It is unfair and irresponsible for Republicans to use the debt limit deadline to force Democrats into an awkward parliamentary procedure, to waste the majority’s floor time or to complicate the lawmaking process. Democrats cooperated to ease the limit during the Trump years, during which the national debt grew by almost $7.8 trillion; Republicans should provide the needed votes now.

If Republicans are as dug in as they appear to be, Democrats must be prepared to go it alone, and soon, to avoid even approaching default. Democrats could change Senate rules to kill the filibuster for debt limit hikes, which would permit debt limit bills to pass with a bare majority, rather than the 60 votes typically required. Or they could use reconciliation, as Republicans demand, which would take time and raise procedural uncertainties that could themselves rattle markets.

Whatever Mr. McConnell and Mr. Schumer decide, it must happen quickly. No more games. The world needs clarity.