US intelligence chief Joseph Maguire, whom Trump replaced on Wednesday with a loyal partisan lacking direct experience in the field, warned lawmakers of Russian interference in a classified briefing last week.
The US intelligence community publicly concluded that Russia intervened in 2016, including by manipulating social media, but Maguire reportedly revealed that Moscow wanted Trump to be re-elected and was meddling in the Democratic Party's primaries.
Trump, who has repeatedly shown irritation at assertions that Russia helped him win the White House, denounced the latest assertions as the work of the rival party.
"Another misinformation campaign is being launched by Democrats in Congress saying that Russia prefers me to any of the Do Nothing Democrat candidates who still have been unable to, after two weeks, count their votes in Iowa," Trump tweeted, referring to the debacle in releasing results from the Democrats' first contest.
"Hoax number 7!"
In Moscow, Kremlin spokesman Dmitry Peskov said the allegations were "like the usual paranoid announcements, which unfortunately will multiply as we get closer to the election."
"Of course, they have nothing to do with the truth," he said.
President Vladimir Putin, however, acknowledged when he met Trump in July 2018 that he supported the populist billionaire's campaign, seeing him as friendlier to Moscow than rival Hillary Clinton.
An extensive report by former FBI chief Robert Mueller found that Russia intervened to back Trump but did not conclude that the Trump campaign conspired with Moscow.
Trump was later impeached in a separate scandal over holding back military aid to Ukraine, which is fighting Russian-backed separatists, to pressure the Kiev government to dig up dirt on a Democratic candidate.
The Senate, led by Trump's Republican Party, acquitted him on February 6, after which the president swiftly removed officials who provided evidence against him.
National security crisis
Former CIA director John Brennan, who served under presidents Barack Obama and George W. Bush, said the United States had entered "a full-blown national security crisis."
"By trying to prevent the flow of intelligence to Congress, Trump is abetting a Russian covert operation to keep him in office for Moscow's interests, not America's," he wrote on Twitter.
Democratic presidential candidate Elizabeth Warren said the 2020 election revelations showed the "enormous" challenge.
"It should worry, all of us, that we now live in an America where one political party seems to think that foreign interference helps them in an election," she said at a CNN forum with voters in Nevada.
Senator Chris Van Hollen denounced both Trump's appointment of a "novice" new intelligence chief and the Senate's blocking of election reform bills backed by Democrats.
"Our intel agencies say Putin is interfering in our election to help Trump," he wrote on Twitter. "What's the definition of treason?"
Representative Eric Swalwell, who serves on the House intelligence committee, tweeted simply: "Agent for Russia."
Trump ally installed as director
Few Republican lawmakers immediately spoke out, although Trump's allies in the past have questioned why Russia would favor him.
While Trump has spoken with admiration about Putin, his administration, staffed with more traditionally Russia-skeptic Republicans, has provided military aid to Ukraine and pushed through sanctions that hit Moscow.
The New York Times, quoting anonymous sources, reported that Trump lashed out not at the purported Russian interference but at Maguire for allowing his staff to brief lawmakers -- particularly Adam Schiff, the House intelligence chief who also led the impeachment drive.
While Maguire would have been obliged to leave next month as he was serving in acting capacity, Trump on Wednesday replaced him with Richard Grenell, the outspoken US ambassador to Germany.
Grenell was an unusual choice for the largely behind-the-scenes job as he has no direct intelligence experience and has made waves by irritating German leaders with his prolific online commentary.
A longtime media commentator, Grenell has played down Russian interference in 2016, saying that Moscow routinely conducts such operations.
While evidence for the latest meddling assertions is classified, Democratic presidential frontrunner Bernie Sanders alleged at a debate Wednesday that Russia could be behind belligerent online remarks by his professed supporters that have drawn condemnation from other candidates.