WASHINGTON (Reuters) - U.S. President Donald Trump will convene his national security advisers on Friday for a high-level meeting on election security, less than four months until Americans vote to determine whether his fellow Republicans maintain control of Congress.

The White House meeting comes amid new accusations of Russian meddling in U.S. elections. On Thursday, Democratic Senator Claire McCaskill was identified as the target of a hacking attempt ahead of the November midterm elections. A top Microsoft official last week said Russian hackers have targeted at least three candidates but did not name them.

Questions persist over whether U.S. states are prepared to thwart cyber attacks even as debate rages over alleged Russian interference in the 2016 presidential election, which Trump won.

The stakes in the midterm elections are high. Republicans hold majorities in both the House of Representatives and the Senate, but if historical patterns hold true, Democrats, as the party out of power, could gain ground in the contests.

U.S. intelligence agencies have concluded that Russia interfered in the 2016 presidential election through a campaign of hacking and disinformation to try to help elect Trump.

The U.S. Special Counsel’s Office is investigating Russian actions and any potential collusion with Trump’s campaign. The probe has so far resulted in multiple indictments and guilty pleas by several Trump associates. Several congressional committees also have opened investigations.

Russia has denied any interference while Trump has denied collusion and blasted the investigation as a “witch hunt,” a sentiment he tweeted again on Friday ahead of his meeting.

Trump has alternated between citing Russian President Vladimir Putin’s denials and saying that he backs the American intelligence community’s findings.

U.S. Special Counsel Robert Mueller earlier this month indicted 12 Russian officers from Russia’s military intelligence agency, the GRU. In February, Mueller charged 13 Russians and three Russian companies in an elaborate conspiracy to interfere in the election.

FILE PHOTO: U.S. President Donald Trump walks from Marine One as he returns to the White House in Washington, U.S., July 26, 2018. REUTERS/Mary F. Calvert

Tom Burt, vice president of customer security and trust at Microsoft Corp, last week told the Aspen Security Forum the company uncovered phishing activity targeting campaign staffers. Microsoft believed the effort was originated by a group associated with GRU officers, he said.

On Thursday, the Daily Beast identified McCaskill, a Missouri Democrat who has been critical of Trump and Putin, as the target of an unsuccessful hacking attempt.

“Russia continues to engage in cyber warfare against our democracy,” McCaskill said on Thursday. “While this attack was not successful, it is outrageous that they think they can get away with this.”

A spokesman for McCaskill, whose tough re-election bid is one of a handful of critical races in the narrowly divided Senate, did not respond to request for comment on Friday.

The United States already has imposed sanctions on Russia, and Congress is considering additional penalties in case U.S. authorities determine the Kremlin has meddled again.

Congress also is grappling with funding to shore up election systems for U.S. states, which conduct elections. U.S. lawmakers earlier approved $380 million to safeguard systems, but this month rejected a Democratic push to provide more funds.

Earlier on Friday, U.S. Defense Secretary Jim Mattis told reporters at the Pentagon the administration was taking steps to protect the elections from interference but did not provide details.

“Rest assured there are actions underway to protect our elections or to expose any external – any, by anybody - external efforts to influence the American public,” Mattis said.

Twenty-one states were targeted in 2016, although there was no evidence that voted were changed, according to DHS, which unveiled its new cyber security strategy earlier this year.

Trump, offering no evidence, this week said he believed Russia will seek to boost Democrats ahead of the November election, rather than target his fellow Republicans.