Top congressional Democrats warned in a cryptic letter they released on Monday that a foreign power was using disinformation to try to interfere in the presidential election and the activities of Congress, and demanded a prompt briefing by the F.B.I. to warn every member of Congress.

While the letter writers, led by Speaker Nancy Pelosi, did not specify the threat, officials familiar with a classified addendum attached to it said the Democrats’ concerns touched on intelligence related to a possible Russian-backed attempt to smear the presidential campaign of former Vice President Joseph R. Biden Jr.

They contend that the Russian-linked information is being funneled to a committee headed by Senator Ron Johnson, the Wisconsin Republican who is investigating Mr. Biden and his son, who was once paid as a board member of a Ukrainian energy company. While neither Mr. Johnson’s inquiry nor much of the information in question is new, the Democrats’ letter is an attempt to call attention to their concern that the accusations are not only unfounded but may further Russia’s efforts to interfere again in the American presidential election.

The warning had echoes of the 2016 campaign. In August of that year, after receiving briefings from the head of the C.I.A. at the time, John O. Brennan, the Senate minority leader at the time, Harry Reid, Democrat of Nevada, publicly warned of a Russian effort to undermine the 2016 elections. Those efforts accelerated as Election Day approached, and this year Ms. Pelosi and other Democrats, including Representative Adam B. Schiff of California, the chairman of the House Intelligence Committee, have vowed to highlight any similar efforts.

In the letter, addressed to Christopher A. Wray, the F.B.I. director, on July 13, the Democrats wrote that they were “gravely concerned, in particular, that Congress appears to be the target of a concerted foreign interference campaign, which seeks to launder and amplify disinformation in order to influence congressional activity, public debate and the presidential election in November.”

Ms. Pelosi was joined by Senator Chuck Schumer of New York, the party’s leader in the Senate, as well as Mr. Schiff and Senator Mark Warner of Virginia, the top Democrat on the Senate Intelligence Committee. Democrats did not ask Republicans to sign on, according to a congressional official familiar with the letter who insisted on anonymity to discuss it.

Patrick Boland, a spokesman for Mr. Schiff’s committee, declined to comment on the content of the intelligence that led to the letter.

Mr. Biden issued a statement on Monday, apparently independently of the congressional Democrats’ request to the F.B.I., that echoed many of the same themes. “I am putting the Kremlin and other foreign governments on notice,” he said, in his most extensive comments on evidence of renewed Russian activity. “If elected president, I will treat foreign interference in our election as an adversarial act that significantly affects the relationship between the United States and the interfering nation’s government.”

He said he would “direct the U.S. intelligence community to report publicly and in a timely manner on any efforts by foreign governments that have interfered, or attempted to interfere, with U.S. elections,” accusing President Trump of ignoring such evidence.

Behind the congressional Democrats’ warning are the efforts of a Ukrainian lawmaker, Andriy Derkach, who was educated in a K.G.B.-backed school and was, until recently, closely aligned with a pro-Russian political faction in Ukraine. In May he released tapes of what he said were fragments of telephone conversations between Mr. Biden, then the vice president, and Ukraine’s president at the time, Petro O. Poroshenko.

The purported tapes did little to change the understanding of the elder Mr. Biden’s engagement with Ukraine’s leadership. Mr. Biden had publicly insisted that aid to Kyiv could be tied to the firing of a Ukrainian prosecutor, whom the United States and European nations had accused of corruption. So far, the accusations emanating from Mr. Derkach and others about Mr. Biden and his son, Hunter, have either been debunked or not been substantiated by any independent sources.

But the accusations, fueled by release of the tapes, led to more online conspiracy theories, which American intelligence officials have warned may well have their origins in Russia. Mr. Johnson has at various moments said he would subpoena evidence surrounding the dealings with Ukraine in the last years of the Obama administration, and The Washington Post reported this month that there was an effort among Ukrainian officials to pass the tapes to Mr. Johnson’s committee.

Mr. Trump has cheered his work. Pressure Mr. Trump put on Ukraine’s leaders last year to investigate some of the same issues ultimately led to his impeachment, and in addition to raising questions about Mr. Biden and his son, Mr. Johnson’s investigation has lent some senatorial legitimacy to Mr. Trump’s claims about the matter.

The investigation focuses on Hunter Biden’s work for a Ukrainian energy firm, Burisma Holdings, and whether his presence and lucrative paycheck were meant to improperly curry favor with the Obama administration. Politico first reported that the Democrats’ concerns were connected to Mr. Johnson’s work.

A spokesman for Mr. Johnson, Austin Altenburg, said the chairman and his staff had already requested briefings from F.B.I. officials, and he accused Democrats of hypocrisy for ignoring recently declassified documents that suggested the F.B.I. may have relied on potentially tainted information in its investigation of ties between Mr. Trump’s campaign and Russia.

The letter from the Democrats asked for the F.B.I. to share what it knows with all lawmakers, “given the seriousness and specificity of these threats.” The Democrats requested a briefing on the matter by the end of the month, when lawmakers are scheduled to leave Washington for several weeks.

Mr. Wray has previously warned that Russia would attempt to interfere in the 2020 election, probably changing the playbook it deployed four years ago.

The letter implied that the lawmakers — who are regularly privy to sensitive intelligence — had seen reporting and analysis by American intelligence agencies that prompted concern, but not necessarily that the information was new. They sent copies to Gina Haspel, the C.I.A. director, and Gen. Paul M. Nakasone, the director of the National Security Agency and U.S. Cyber Command.

Despite Mr. Trump’s unease discussing Russian efforts to influence elections — and his refusal to say he would stop any foreign assistance to his own campaign — top national security officials working for him at the Department of Homeland Security and intelligence agencies have insisted that securing the vote in November is a top concern.

General Nakasone described in a speech on Monday “our No. 1 goal, our No. 1 objective at the National Security Agency and U.S. Cyber Command: a safe, secure and legitimate 2020 elections.”

“We are going to know our adversaries better than they know themselves,” he said.

He made no specific reference to current investigations of Russia, though he had directed an effort to neutralize Russia’s Internet Research Agency and some Russian intelligence operatives during the 2018 midterms. Mr. Trump recently took credit for that effort.

An F.B.I. spokeswoman acknowledged that the bureau had received the letter, but declined to comment further.

Mr. Wray is in a precarious position. Although appointed by Mr. Trump, he is considered suspect by the president. And while Mr. Trump has complained that the Russia and Ukraine investigations that led to Mr. Trump’s impeachment — which also focused on his efforts to obtain incriminating information about Mr. Biden and his son — was a “hoax,” the Russian nexus with Ukrainian officials remains the subject of federal investigators.

Now Republicans have come back with their own investigations, focused on Hunter Biden and Burisma. In the course of its investigation, the Senate Homeland Security and Governmental Affairs Committee has relied extensively on a former Ukrainian official, Andrii Telizhenko. Mr. Telizhenko has insisted that he is not trying to interfere in the presidential election, and that he would turn the information he received over to unspecified American officials.

In March, however, F.B.I. officials warned lawmakers in a briefing that it had concerns that Mr. Telizhenko was a conduit for Russian disinformation about the Bidens and claims that Ukraine conspired to help Democrats in the 2016 election. After the briefing, lawmakers in both parties pressured Mr. Johnson to abandon a vote to subpoena Mr. Telizhenko, and he turned his attention to other possible sources of information.

Senate Democrats on the committee, who view its work as an attempt to simply smear Mr. Biden ahead of election, wrote to Mr. Johnson again last week demanding that he schedule a briefing with the F.B.I.’s foreign influence task force and other intelligence agencies about his work.