WASHINGTON — The Democratic leadership in the Senate asked President Obama on Friday to take “any lawful steps” available to avoid a default on the nation’s debt if Republicans continue to press their demand that an increase in the government’s borrowing limit be accompanied by spending cuts of the same magnitude.

“In the event that Republicans make good on their threat by failing to act, or by moving unilaterally to pass a debt-limit extension only as part of unbalanced or unreasonable legislation, we believe you must be willing to take any lawful steps to ensure that America does not break its promises and trigger a global economic crisis — without Congressional approval, if necessary,” wrote Senators Harry Reid of Nevada, Richard J. Durbin of Illinois, Charles E. Schumer of New York and Patty Murray of Washington.

The letter signaled an escalation in the war of words over the federal debt ceiling, which has already technically been breached, leaving the Treasury Department scrambling to meet the government’s domestic and foreign obligations. Lawmakers believe the bookkeeping flexibility will be exhausted by Feb. 15, at which time Washington would have to either default on its debt or shut down major expenditures.

Already, liberal policy experts have been trying to rally support for measures to go around the Republican blockade, from declaring that the Constitution’s 14th Amendment gives the president unilateral authority to raise the debt ceiling to calling for the minting of a trillion-dollar platinum coin that would be used to pay the nation’s debts.