WASHINGTON—A bipartisan think tank warned on Friday the U.S. government could start missing payments on its bills as soon as Dec. 21 if Congress fails to raise the debt limit, as top Democrats and Republicans sought a path around such a financial calamity. The Bipartisan Policy Center’s projection, based on updated official data on tax receipts and government spending, underscores the mounting pressure on President Joe Biden’s Democratic Party to find a way to raise the statutory $28.9 trillion debt limit and avoid the heavy economic repercussions that could come with missed payments. Congress passed legislation on Thursday to fund the government through mid-February, averting the risk of a partial government shutdown for now. But the task of bridging partisan differences over the debt limit could prove more difficult and pose risks that are far more severe. “Congress would be flirting with financial disaster if it leaves for the holiday …