“It’s time that we got serious on a bipartisan basis to try to work this out and not have the kind of chaos that goes along with our inability to come together on these kind of important issues,” said Senator Mitch McConnell, Republican of Kentucky and the majority leader, at a news conference on Tuesday.

The federal government has already run a $747 billion budget deficit for the 2019 fiscal year, which ends in September — a 23 percent increase from the year before. That is an unusually large increase given the strength of economic growth.

Total personal and corporate income tax levels are down slightly from the previous year, Treasury Department statistics show. Federal spending has risen — particularly for national defense and health care programs — and so have the interest costs on the growing national debt. Those trends reflect Washington’s free-spending ways and the rising costs of an aging population as the baby boom generation draws Social Security and Medicare benefits.

Those dynamics have yet to prompt a renewed interest in the deficit, but they are intensifying pressure on lawmakers to raise the debt limit — or risk economic catastrophe if they do not.

A 2018 budget deal in Congress suspended the limit, which constrains the amount of money the government may borrow to continue paying its bills, until this past March 1. Once that deadline passed, the Treasury Department was forced to begin employing “extraordinary measures” in order to maintain the borrowing needed to keep paying soldiers, issuing Social Security checks and otherwise funding the government.

Those measures include premature redemption of Treasury bonds owned by federal employees’ retirement accounts, borrowing cash set aside to smooth exchange rate fluctuations and stopping contributions to some government pension funds.

Mr. Mnuchin’s letter is a warning that those sleight-of-the-balance-sheet efforts cannot last forever. When the Treasury Department runs out of them, it will not have enough money coming in to pay its spending obligations. That would necessitate either sharp and immediate spending cuts, or a government default on some of its obligations — not paying workers, contractors, lenders or citizens it owes money to.