By Dustin Volz

WASHINGTON (Reuters) - Democrats in Congress, including party Senate leader Harry Reid, asked the FBI to investigate concerns that the Russian government may be attempting to undermine the U.S. presidential election through cyber attacks that could include tampering with voting results.

"The prospect of a hostile government actively seeking to undermine our free and fair elections represents one of the gravest threats to our democracy since the Cold War," Reid said in a letter to FBI Director James Comey.

Reid's letter, dated Saturday, was first obtained by The New York Times and shared on its website on Monday.

It was followed on Tuesday by a letter from four Democrats asking Comey to assess whether campaign officials working for Republican presidential candidate Donald Trump may be colluding with Russian interests to carry out recent hacks against the Democratic National Committee and Democratic Congressional Campaign Committee in order to "interfere with the U.S. presidential election."

That letter was signed by Representatives Elijah Cummings, John Conyers, Elliot Engel and Bennie Thompson, each of whom serves as the top Democrat on a different congressional committee. Republicans control both the House of Representatives and the Senate.

Comey, speaking at a cyber security conference on Tuesday, declined to give details about what the Federal Bureau of Investigation was probing in connection with the political hacking but indicated the agency was closely watching what foreign countries are doing.

"We take very seriously any effort by any actor, ... especially nation states, that moves beyond the collection of information ... and offers the prospect of an effort to influence the conduct of affairs in our country, whether that is an election or something else," he said.

The two letters follow a spate of hacking attacks targeting U.S. political databases, including some that officials and cyber security experts have blamed on hackers working for the Russian government. Kremlin officials have denied that.

The FBI examined breaches in voter registration databases in Illinois and Arizona but did not specify who might have been behind it.

Reid said that the threat of Russian government tampering in the election was "more extensive than widely known and may include the intent to falsify official election results."

He also voiced concerns about possible Russian government efforts to manipulate Trump's campaign ahead of the Nov. 8 election, and to use it as a vehicle to advance the interests of Russian President Vladimir Putin.

(Reporting by Dustin Volz and David Alexander; Editing by Frances Kerry and Jonathan Oatis)