Sens. Amy Klobuchar Amy KlobucharBattle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates Klobuchar: GOP can't use 'raw political power right in middle of an election' MORE (D-Minn.) and Jeanne Shaheen Cynthia (Jeanne) Jeanne ShaheenSenate Democrats introduce bill to sanction Russians over Taliban bounties Trump-backed candidate wins NH GOP Senate primary to take on Shaheen Democratic senator urges Trump to respond to Russian aggression MORE (D-N.H.) sent a letter Wednesday to three election equipment vendors to ask whether they have shared information about their machines with Russian entities.

The senators wrote to Election Systems & Software, Dominion Voting Systems Inc. and Hart InterCivic Inc. to ask if the companies had shared the source code, software or other sensitive details about their machines with Russians.

“Foreign access to critical source code information and sensitive data continues to be an often overlooked vulnerability. Further, if such vulnerabilities are not quickly examined and mitigated, future elections will also remain vulnerable to attack,” the senators wrote.

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The senators also asked the companies what steps they've taken to upgrade their technology in light of ongoing cybersecurity threats. Lawmakers have expressed concerns that Russia will seek to interfere in the 2018 midterm elections.

“The 2018 election season is upon us. Primaries have already begun and time is of the essence to ensure any security vulnerabilities are addressed before 2018 and 2020,” Klobuchar and Shaheen wrote.

The lawmakers cited a Reuters report from January that a number of major technology providers let Russian authorities probe their software for vulnerabilities that could be exploited by hackers. That software is used by various agencies of the U.S. government.

U.S. officials have said Moscow targeted 21 states’ voting systems two years ago but did not succeed in changing votes. The Department of Homeland Security also has said there is no evidence to suggest that votes or voter rolls were altered.

However, lawmakers and members of the Trump administration have warned in recent months that they expect Russia will attempt to influence the 2018 elections, pointing to Moscow's influence campaign aimed at swinging the 2016 presidential race.

President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE said Tuesday that the U.S. will work to counteract any Russian meddling efforts "very strongly."

Several administration officials have said they have not explicitly been directed to counteract Russian efforts, and others have said U.S. efforts must be more robust.