The House on Friday passed a two-year reauthorization of an expiring warrantless surveillance law known as Section 702, reversing course after the bill collapsed days earlier when former President Donald J. Trump urged his allies to “kill” it.

But disappointing privacy advocates, the House narrowly rejected a longstanding proposal to require warrants to search for Americans’ messages swept up by the program.

Here is a closer look.

What is Section 702?

It is a law that allows the government to collect — on domestic soil and without a warrant — the communications of targeted foreigners abroad, including when those people are interacting with Americans.

Under that law, the National Security Agency can order email services like Google to turn over copies of all messages in the accounts of any foreign user and network operators like AT&T to intercept and furnish copies of any phone calls, texts and internet communications to or from a foreign target.

Section 702 collection plays a major role in the gathering of foreign intelligence and counterterrorism information, according to national security officials.

Why was Section 702 established?

After the attacks of Sept. 11, 2001, President George W. Bush secretly ordered a warrantless wiretapping program code-named Stellarwind. It violated the Foreign Intelligence Surveillance Act of 1978, or FISA, which generally required a judge’s permission for national security surveillance activities on domestic soil.

The main rationale was that when Congress enacted FISA, lawmakers had relied on geography to require warrants for domestic wiretapping while keeping overseas spying unfettered. But technological advances — the internet and fiber-optic lines — made foreigners’ messages available on domestic networks, where FISA’s warrant rule applied.

Stellarwind was based on a disputed assertion of executive power. Congress later legalized a form of that program by carving out an exception, now known as Section 702, to the warrant rule in FISA.

Why is Section 702 controversial?

Privacy advocates have criticized Section 702 because it sometimes enables the government to collect Americans’ messages without a court order. While the law forbids using Section 702 to target Americans, when a foreign target communicates with an American, the government incidentally collects that American’s messages to and from its target.

Analysts at several agencies, including the F.B.I., can search the repository of messages by using Americans’ identifiers — like names, Social Security numbers, passport numbers, phone numbers and email addresses — as search terms. Critics call such queries a “backdoor search loophole” to the Fourth Amendment and have long wanted Congress to require the government to get warrants before seeking out Americans’ private communications.

The repeated disclosure in recent years that F.B.I. analysts have violated standards for when they search for information about Americans has only heightened tensions. That includes analysts who improperly queried for information about Black Lives Matter protesters and people suspected of participating in the Jan. 6, 2021, attack on the Capitol. The F.B.I. has since tightened restrictions to reduce the risk of such misuses.

What would happen if Section 702 lapses?

Even if the law were to expire next Friday, the program could continue operating until April 2025 because last week the FISA court granted a government request authorizing it for another year. Under the law, surveillance activity can continue so long as there are active court orders allowing it, even if the underlying statute expires.

Even so, the intelligence community has urged Congress to reauthorize the program before it enters that sort of legal limbo, raising the possibility that providers might balk at continuing to cooperate and leading to gaps in collecting intelligence. It is also unlikely the FISA court could order new companies to start participating in the program if the underlying statute has expired.

Why did Trump want to kill it?

For political reasons that are incoherent as a matter of law and policy, as the House prepared to take up the legislation this week, Mr. Trump weighed in, urging supporters to “KILL FISA, IT WAS ILLEGALLY USED AGAINST ME, AND MANY OTHERS. THEY SPIED ON MY CAMPAIGN!!!”

Mr. Trump’s blast was part of his yearslong effort to stoke grievances about national security agencies. His dissatisfaction stems from an inspector general’s finding that the F.B.I. botched applications for FISA warrants to target a former campaign adviser as part of the investigation into ties between his 2016 campaign and Russia. (A follow-up report found systemic sloppiness in unrelated applications.)

But Mr. Trump was mixing up which part of FISA he takes issue with from the Russia investigation. (The part in question involves warrants and is not expiring; it was not, in fact, Section 702.) Still, after his plea, 19 hard-right Republicans blocked the House on Wednesday from taking up the legislation — delivering a blow to the national security officials who say Section 702 is a crucial tool to protect the country.

How did Speaker Johnson resuscitate the bill?

Speaker Mike Johnson cut the extension to two years from five, allowing the hard-right Republicans to claim victory. Among other things, if Mr. Trump wins the 2024 election, he would be in office when the law next comes up for renewal. All 19 of the Republicans who had blocked consideration of the bill on Wednesday voted to bring it up on Friday.

What happened to the warrant proposal?

An amendment to add it to the bill failed on a dramatic 212-to-212 vote — under House rules, a tie means a measure fails.

National security officials from the Biden administration had lobbied Congress heavily not to pass the amendment. They argued that it would cripple the program because they typically use it before there is enough evidence to meet a standard of probable cause for a warrant, like early in investigations when they are trying to learn more about a phone number or an email account found to be in contact with a suspected foreign spy or terrorist.

The close vote deeply disappointed privacy advocates, who instead see a warrant requirement as necessary to uphold constitutional principles. They came much closer to succeeding this cycle than they had in previous ones.

What other changes would the bill make?

The bill would codify as a matter of law the changes the F.B.I. has already made to reduce the risk of improper queries, add new reporting requirements, and sharply cut down on the number of F.B.I. officials with access to the repository of raw information.

The House on Friday approved several amendments that would make other significant modifications, including by allowing the Section 702 program to be used to gather intelligence on foreign narcotics trafficking organizations and to vet potential foreign visitors to the United States.

The House also approved a measure — one whose effect is murky, but that the Justice Department had requested and privacy advocates have raised alarms about — to expand the type of companies with access to foreign communications that can be required to participate in the program. The technical details of why the government wants that change are classified.

What happens next?

The House must formally transmit the bill to the Senate. There is sure to be further debate in the upper chamber, where privacy-minded lawmakers like Senator Ron Wyden, Democrat of Oregon, are likely to try again to attach new limits to the legislation.