House Judiciary Committee Democrats – in a letter sent from Ranking Member Jerrold Nadler (D-NY) to Chairman Bob Goodlatte (R-VA) – are calling for "immediate hearings examining the U.S. election infrastructure" in the wake of news that Russian hackers targeted 21 states during the 2016 election and managed to penetrate a small number of voter registration rolls.
From the committee's press release:
The letter asks the Majority for emergency hearings to look into potential vulnerabilities of our election infrastructure, foreign threats posed, and to examine what steps the Department of Justice, Homeland Security, and the State Department may or may not be taking to ensure the integrity of future state and federal elections.
(...)
With the first primary election in March 2018, Judiciary Republicans have taken no action to address the ongoing threat Russia poses to the U.S. federal election systems—despite pledging to do so when adopting the Committee’s oversight plan in February 2017.
Unfortunately, the Department of Justice appears to have taken little—if any—action to secure our election systems going forward. When he appeared before our Committee, Attorney General Sessions was asked if the Department is at least reviewing the laws on the books to see what additional authority he might need to secure our election infrastructure. He gave a completely unsatisfactory answer: “We have discussed those matters, but no completion has been done. ... We are not anywhere near where I would like us to be, yet.” He later agreed he had not, in fact, ordered any review of relevant federal law—ignoring President Trump’s executive order to strengthen federal networks and infrastructure. Although the Attorney General promised our members a briefing on the topic, but that was more than nine weeks ago. The Department of Justice has not yet responded to the request we sent him on December 1, 2017.
Democrats are also angry with the White House for not imposing congressionally mandated sanctions on Russia in retaliation for its 2016 actions.
House Democrats demand election security hearings (Politico)
HOUSE JUDICIARY DEMOCRATS CALL FOR EMERGENCY HEARINGS AFTER ADMINISTRATION OFFICIAL REVEALS RUSSIA HACKED U.S. VOTER ROLLS IN 2016 (press release)
Full Letter (pdf)