Rep. Adam Schiff, the ranking Democrat on the House Intelligence Committee, said Sunday that President Barack Obama should have taken action once he learned of Russian meddling in the U.S. presidential election.

Schiff, D-Calif., in an appearance on CNN’s “State of the Union,” said the American people “needed to know” high-level Russian officials were attempting to interfere with the U.S. election process.

"I think the Obama administration should have done a lot more when it became clear that not only was Russia intervening, but it was being directed at the highest levels of the Kremlin," Schiff said.

President Donald Trump, who for months has rejected the notion of Russian interference, appeared finally to acknowledge the possibility in a series of tweets Saturday.

"I just heard today for the first time that Obama knew about Russia a long time before the election, and he did nothing about it," Trump said in the interview broadcast Sunday by Fox News. "But nobody wants to talk about that."

Read: Russia Inquiry Just Beginning, Schiff Says

Trump has called the investigation into whether his campaign colluded with Russian officials a “witch hunt,” and former FBI Director James Comey testified before the Senate Intelligence Committee the president asked him to drop the investigation into former national security adviser Michael Flynn’s contacts with Russians.

U.S. intelligence agencies concluded Russian hackers stole embarrassing emails from the Democratic National Committee and leaked them to WikiLeaks. They also planted fake news stories on social media, one of which implicated top Democrats in a child trafficking ring run out of a Washington pizza parlor.

The Washington Post reported Friday the interference was directed by Russian President Vladimir Putin and aimed at defeating Democratic nominee Hillary Clinton’s presidential bid. The FBI opened an investigation July 22, and some lawmakers were briefed in September, but the White House did not disclose the plot publicly until after the election.

Read: Susan Rice Says Vladimir Putin Is Lying

Schiff said Obama should have done “a lot more” and disclosed the interference much earlier.

Sen. Joe Manchin, D-W.Va., agreed in an appearance Sunday on CBS’s “Face the Nation.”

“I don't know why there's so much skepticism, not believing what the intelligence community is telling you. I do, and I have found it to be extremely beneficial to me to make decisions with,” Manchin said.

“We knew there was a serious problem, and it was verified. I don’t know why we don’t go forward and impose more sanctions. …

"What was known back in August and once it was verified and cross-checked should have been made public. It should have been made public, OK? That wasn't done," Manchin said.

Manchin said treating Putin as “an ally and a friend is wrong.”

"Russia is not our friend. … I don't look at him as a friend. I don't look at Russia. And I am very skeptical of what they're doing, their intentions. There are a lot of good people in Russia that don't have any say whatsoever," Manchin said

Putin has attributed the interference to patriotically minded Russian hackers, likening them to artists.

"[Artists] may act on behalf of their country, they wake up in good mood and paint things. Same with hackers, they woke up today, read something about the state-to-state relations. If they are patriotic, they contribute in a way they think is right, to fight against those who say bad things about Russia," Putin told reporters at the St. Petersburg Economic Forum earlier this month.

In March, Putin flatly denied involvement, saying, “Read my lips: No.”

The Justice Department appointed former FBI Director Robert Mueller as a special counsel to direct the investigation into Russian interference in the election. Trump, however, said the inquiry “hurts our country,” and his surrogates in recent weeks have been attempting to paint Mueller’s investigation as a partisan effort to damage the Trump presidency.

Mueller, who is known for staying above the political fray, has appointed more than a dozen lawyers to conduct the inquiry, including Andrew Weissmann, who led the Enron investigation.