American intelligence officials issued a public warning on Friday that China was “expanding its influence efforts” in the United States ahead of the presidential election, along with Russia and Iran, but Democrats briefed on the matter said the threat was far more urgent than what the administration described.

The warning came from William R. Evanina, the director of the National Counterintelligence and Security Center, in a statement 100 days before Americans go to the polls. “We’re primarily concerned with China, Russia and Iran — although other nation-states and nonstate actors could also do harm to our electoral process,” the statement said.

The warning about China came at a moment of extraordinary tension between Beijing and Washington, only days after the United States indicted two Chinese hackers on charges of stealing intellectual property, including for the country’s main intelligence service, and evicted Chinese diplomats from their consulate in Houston.

The intelligence warning on Friday did not accuse the Chinese of trying to hack the vote; instead it said they were using their influence “to shape the policy environment in the United States” and to pressure politicians “it views as opposed to China’s interests.”

Russia, the warning said, was continuing to “spread disinformation in the U.S. that is designed to undermine confidence in our democratic process,” and it described Iran as an emerging actor in election interference, seeking to spread disinformation and “recirculating anti-U.S. content.”

The statement was short on details, reminiscent of the vague warnings that the director of national intelligence turned out starting in October 2016 that, in retrospect, failed to seize the attention of officials and voters before the last presidential election.

In a statement issued a few hours later, Speaker Nancy Pelosi was joined by the Senate Democratic leader, Senator Chuck Schumer of New York, and two key Democrats on intelligence oversight committees, Senator Mark Warner of Virginia and Representative Adam B. Schiff of California, in saying that the descriptions of malign activity were “so generic as to be almost meaningless.”

Mr. Evanina’s statement, said the four Democrats, who earlier called on the F.B.I. to give a briefing on disinformation campaigns to the entire Congress, “does not go nearly far enough in arming the American people with the knowledge they need about how foreign powers are seeking to influence our political process.”

Their letter was particularly critical of the description of Russian activity, the most politically delicate topic because of President Trump’s own unwillingness to acknowledge Russia’s actions four years ago. The Democrats wrote that “to say without more, for example, that Russia seeks to denigrate what it sees as an anti-Russia ‘establishment’ in America is so generic as to be almost meaningless.”

Mr. Schiff, who is the chairman of the House Intelligence Committee, said Friday on MSNBC that he had been “urging Bill Evanina and others in the intelligence community to level with the American people about what’s going on.” He said the warning gave “a false sense of equivalence between what Russia is doing, what China is doing, what Iran is doing.”

Mr. Schiff and the other three authors of the letter have been briefed extensively on the intelligence, and thus are prohibited from violating classification rules by describing what they have seen.

But Mr. Schiff, a frequent target of harsh criticism from Mr. Trump because he was the Democrats’ manager in the impeachment trial in the Senate, added, “I think that our adversaries, in particular the Russians, are going to amplify the false messages that the president is putting out about, ‘Well, you can’t trust absentee ballots,’ even though that’s how the president votes.”

Some intelligence officials expressed surprise at the lawmakers’ letter and insisted they were not trying to play down the threat of interference from Moscow or signal that China was a greater challenge. They said Mr. Evanina’s statement was meant to be the beginning of a series of public statements, according to an official from the Office of the Director of National Intelligence.

The official said the statement did not play down the threat of Russian interference, but lawmakers had to understand that the 2020 contest would be different from 2016’s.

It is unclear whether those statements, however, deter further action by American adversaries. But it is clear that 2020 will not be the same as 2016 — the Russians know that they cannot use the same playbook, and Iran and China both seem poised to play a greater role.

The question is whether they will be on the same side, or working against each other.

After the 2016 election, American intelligence assessments concluded that the Russians ultimately intervened on Mr. Trump’s behalf. But this year, Republicans and Democrats who have reviewed the intelligence have come to different assessments about whether Russia hopes to swing the election to Mr. Trump, or if President Vladimir V. Putin is simply intent on eroding confidence in the American electoral system.

The threat of Iran to the election is harder to judge. Senior American officials said it was intent on trying to hurt Mr. Trump’s re-election campaign. Some believe Iran would stage attacks on oil shipping this fall, to try to cause economic calamity. But with the global economy already in turmoil from the pandemic, Iran’s room to try to influence the election through such attacks may be more limited.

China is not new to presidential elections. In 2008, intelligence officials warned the campaigns of both Barack Obama and John McCain that Chinese hackers had penetrated their campaign computer systems. But that was intelligence gathering, it appears, not an effort to influence the outcome in the way Russia tried eight years later.

This year, intelligence officials do not believe China will try the same kind of brazen techniques Moscow has employed. Instead, intelligence officials said Friday, China is playing a long game, trying to cultivate local politicians who may ultimately win election to Congress.

Mick Baccio, a former information security official with Pete Buttigieg’s campaign, said that with large numbers of absentee ballots and a potentially long counting period, foreign interference could intensify in November. As votes are being counted, foreign powers could seek to undermine confidence in the vote.

The 2020 election “is like every disaster movie sequel rolled into one,” said Mr. Baccio, now an adviser with the cybersecurity firm Splunk. “The postelection period is what I am most concerned about. The window of time where we are uncertain, that is when they will drop their madness.”

The intensified work by China, Russia, Iran and others provides a major challenge to the campaigns.

“Nothing is unhackable,” Mr. Baccio said. “You raise the bar as best you can. You identify your crown jewels of data and you lock it down the best you can.”

In recent weeks, intelligence officials have briefed lawmakers on Capitol Hill about election interference threats from China, according to American officials.

Those classified briefings included warnings about how the Chinese government was trying to influence the broader political debate in the United States as well as in Congress.

F.B.I. and intelligence officials have warned lawmakers about China’s so-called mask diplomacy, in which Beijing’s diplomats have made donations of personal protective equipment useful to fight the coronavirus pandemic and have demanded support in return.

A Chinese diplomat had asked a Wisconsin state Republican lawmaker to pass a resolution lauding China’s work to fight the virus. The state senator refused and instead introduced a measure criticizing the Chinese Communist Party.

Until now, China has focused on local and congressional races and was less interested in influencing the presidential campaigns, officials said. Local influence campaigns are less likely to receive national attention, and are therefore more likely to succeed, officials said.

But those localized campaigns could influence national or presidential-level politics, current and former intelligence officials said. “Beijing recognizes its efforts might affect the presidential race,” Mr. Evanina wrote in his warning on Friday.

Beijing has been conducting cybersurveillance of the presidential campaigns and related political efforts, said an intelligence official, and it could decide in the coming weeks to expand its influence campaign to try to influence presidential politics more directly.