The Trump administration has decided that the National Security Agency and the F.B.I. can lawfully keep operating their warrantless surveillance program even if Congress fails to extend the law authorizing it before an expiration date of New Year’s Eve, according to American officials.

National security officials have implored Congress for the past year and a half to extend the legal basis for the program, Section 702 of the FISA Amendments Act, before it lapses at the end of the month. They portrayed such a bill as the “top legislative priority” for keeping the country safe.

But with Congress focused on passing a major tax cut and divided over what changes, if any, to make to the surveillance program, lawmakers may miss that deadline. Hedging against that risk, executive branch lawyers have now concluded that the government could lawfully continue to spy under the program through late April without new legislation.