By Dustin Volz

WASHINGTON (Reuters) - A senior Democratic lawmaker said Sunday he had "no doubt" that Russia was behind recent hacking attempts targeting state election systems, and urged the Obama administration to publicly blame Moscow for trying to undermine confidence in the Nov. 8 presidential contest.

The remarks from Representative Adam Schiff, the top Democrat on the intelligence committee in the U.S. House of Representatives, come amid heightened concerns among U.S. and state officials about the security of voting machines and databases, and unsubstantiated allegations from Republican candidate Donald Trump that the election could be "rigged."

"I have no doubt [this is Russia]. And I don’t think the administration has any doubt," Schiff said during an appearance on ABC's "This Week."

Schiff's call to name and shame the Kremlin came a week after Trump questioned widely held conclusions made privately by the U.S. intelligence community that Russia is responsible for the hacking activity.

"It could be Russia, but it could also be China," Trump said during a televised debate with Democratic candidate Hillary Clinton. "It also could be somebody sitting on their bed that weighs 400 pounds."

On Saturday, Homeland Security Secretary Jeh Johnson said hackers have probed the voting systems of many U.S. states but there is no sign that they have manipulated any voting data.

Schiff said he doubted hackers could falsify vote tallies in a way to affect the election outcome. Officials and experts have said the decentralized and outdated nature of U.S. voting technology makes such hacks more unlikely.

But cyber attacks on voter registration systems could "sow discord" on election day, Schiff said. He further added that leaks of doctored emails would be difficult to disprove and could "be election altering."

The National Security Agency, FBI and DHS all concluded weeks ago that Russian intelligence agencies conducted, directed or coordinated all the major cyberattacks on U.S. political organizations, including the Democratic National Committee, and individuals, a U.S. official who is participating in the investigations said on Sunday.

    However, the official said, White House officials have resisted naming the Russians publicly because doing so could result in escalating cyberattacks, and because it is considered impossible to offer public, unclassified proof of the allegation.

Schiff and Senator Dianne Feinstein, the top Democrat on the U.S. Senate intelligence committee, said last month they had concluded Russian intelligence agencies were "making a serious and concerted effort to influence the U.S. election."

(Reporting by Dustin Volz and John Walcott; Editing by Nick Zieminski)