Washington (CNN) -- The Obama administration is expanding its push to stop what it calls "restrictive state laws" over the ability of voters to cast ballots.
The Justice Department said it was filing legal briefs in support of separate, private lawsuits against Wisconsin and Ohio.
"These filings are necessary to confront the pernicious measures in Wisconsin and Ohio that would impose significant barriers to the most basic right of our democracy," said Attorney General Eric Holder. "We will keep using every available tool at our disposal to guard against all forms of discrimination, to prevent voter disenfranchisement, and to secure the rights of every citizen.”
A lawsuit by the NAACP says the Ohio law unfairly restricts early voting and same-day registration. The state holds a special election next Tuesday, in addition to regular balloting in November.
Ohio reduced early voting by one week, and eliminated the so-called "golden week" period when voters were able to register then cast a ballot the same day.
In Wisconsin, a federal judge in April tossed out the state's voter identification law, after opponents claimed it had a discriminatory effect of many Hispanic and African-American voters.
State officials said Act 23 was minimally burdensome, and necessary to stop voter fraud and promote voter confidence.
Civil rights groups claimed about 300,000 voters in Wisconsin lacked the necessary identification to cast a ballot.
There was no immediate reaction from officials in Wisconsin or Ohio to Tuesday's legal action by the federal government.
The Justice Department's latest involvement follows pending federal lawsuits it had filed in Texas and North Carolina over voting laws.
The aggressive legal action is part of the administration's effort to blunt the impact of a 2013 Supreme Court ruling that struck down a key part of the landmark Voting Rights Act, weakening federal oversight of states and counties with a past history of discrimination at the polls.
Holder has since strongly and repeatedly criticized the high court's ruling, urging Congress to pass new legislation to restore the Justice Department's oversight powers.
In a September 2013 speech, Holder called the court's action "a deeply flawed decision that effectively invalidated a cornerstone of American civil rights law."
CNN Justice Reporter Evan Perez contributed to this report