Senate Democratic leaders urged President Barack Obama on Friday to be prepared to lift the debt ceiling “without congressional approval” if Republicans block legislation and default is imminent.

The letter from Majority Leader Harry Reid of Nevada and other Democratic leaders signaled they are taking seriously warnings from Republicans who have said they might be willing to see the government shut down rather than allowing a debt ceiling increase without concessions from Obama on spending cuts.

The letter also signed by Majority Leader Richard J. Durbin of Illinois, Charles E. Schumer of New York and Patty Murray of Washington counters the GOP threat with an equally tough stance, saying the president should “take any lawful steps” needed to avoid default.

Senate Minority Leader Mitch McConnell, R-Ky., called the Democratic leaders’ position “not only the height of irresponsibility, but also a guarantee that our national debt crisis will only get worse.”

The government hit the debt celing at the end of December, but the Treasury has said that “extraordinary measures” will delay a crisis until about the end of February.

House Speaker John A. Boehner, R-Ohio, has said he will push for a dollar in spending cuts for every new dollar in borrowing authority, a formula he came up with during last year’s debt ceiling debate. “The American people will not tolerate an increase in the debt limit without spending cuts and reforms,” Boehner aide Michael Steel said in response to the Democrats’ letter. “Washington Democrats must stop spending money we don’t have.”

Some GOP lawmakers have said the Treasury Department could prioritize debt payments to minimize the impact of default. But Treasury officials say they have no such authority.

The Senate Democratic leaders told Obama not to assume Republicans will relent. “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,” they wrote.

Some Democrats, including House Minority Leader Nancy Pelosi, say the president should invoke authority under the 14th Amendment to the Constitution to continue spending money authorized by Congress.

Others have urged the administration to mint a $1 trillion denomination coin, which could be deposited in the Federal Reserve to offset obligations. The Treasury secretary has legal authority to mint platinum coins of any denomination. Asked about the coin idea Thursday, White House Press Secretary Jay Carney did not rule it out. But he said Congress should increase the debt limit.

“There is no plan B. There is no backup plan,” Carney said. “The only option is for Congress to do its job.”

Carney also said the administration stands by its opinion that the 14th Amendment — which includes a clause saying the public debt shall not be questioned — does not give the president authority to ignore the debt limit. “Our position on the 14th Amendment has not changed. Congress has the responsibility and the sole authority to raise the debt ceiling,” he said.

In their letter, the Senate Democrats suggested Obama might have to take what would be an explosive action because of the inability of Boehner and McConnell to control their conferences. “It is hard to imagine that the speaker and Leader McConnell would really follow through on their threat to let our nation default on its debts. They are responsible leaders who know better,” the Democratic senators wrote. “Sadly, some of their Republican colleagues do not.”