By MAGGIE HABERMAN and AMY CHOZICK

Democrats allied with Hillary Rodham Clinton are mounting a nationwide legal battle 17 months before the 2016 presidential election, seeking to roll back Republican-enacted restrictions on voter access that Democrats say could, if unchallenged, prove decisive in a close campaign.

The court fights began last month with lawsuits filed in Ohio and Wisconsin, presidential battleground states whose governors are likely to run for the Republican nomination. Now, Democrats are attacking a host of measures, including voter identification requirements that they consider onerous, time restrictions imposed on early voting that they say could make it difficult to cast ballots the weekend before Election Day, and rules that could nullify ballots cast in the wrong precinct.

The effort, which is being led by a lawyer whose clients include Mrs. Clinton’s campaign, reflects an urgent practical need, Democrats say: to get litigation underway early enough so federal judges can be persuaded to intervene in states where Republicans control legislatures and governor’s offices. But Republicans dismiss it as little more than a publicity gambit to energize minority voters in support of Democratic candidates.

Hillary Rodham Clinton will make voter access a focus. Credit Isaac Brekken for The New York Times

A similar lawsuit was begun last year in North Carolina. Other potential fronts in the pre-emptive legal offensive, Democrats say, could soon be opened in Georgia, Nevada and the increasingly critical presidential proving ground of Virginia.

Almost all of those states have growing African-American or Hispanic populations, groups crucial to Mr. Obama in 2012 but whose voting rights Democrats say could be impinged next year, damaging the party’s prospects.

Democrats seeking office at every level in 2016 could gain if lawsuits in their states are successful. But Mrs. Clinton, who has a wide lead in public polls, is certain to benefit if she is the party’s nominee.

On Thursday, Mrs. Clinton will tackle the issue of voter access head-on at Texas Southern University, a historically black college in Houston, in a speech that will tie voting rights to broader civil rights issues. She is expected to condemn the Supreme Court’s 2013 ruling striking down an important provision of the Voting Rights Act, which opened the way for states to pass increasingly restrictive laws requiring voters to present government-issued photo identification at the polls, and call on Congress to re-enact parts of the law.

Mrs. Clinton is also expected to single out laws in Texas and in North Carolina, Ohio and Wisconsin that voting rights groups say limit participation, especially among minorities, the poor and younger voters, who disproportionately cast their ballots for Democrats.

And she will criticize her potential Republican rivals, including Gov. Scott Walker of Wisconsin and the former governors Jeb Bush of Florida and Rick Perry of Texas, whose states have enacted laws that put more requirements on voters. She will also propose a nationwide early-voting standard of at least 20 days before an election, call for an increase in online voter registration and restate her position that former felons should have their voting rights restored.

For Mrs. Clinton, such speeches can do much to reinforce her liberal credentials while reassuring black voters who supported Mr. Obama that she is on their side.

In the same way, even if the lawsuits are unsuccessful, Democratic strategists say, they serve an important political purpose for Democrats by highlighting Republicans’ responsibility for the statutes that are being challenged. Democrats will also remind their base that Republicans have tried to mobilize their own core voters by warning, as Mitt Romney did in 2012, against the possibility of widespread voter fraud by Democrats.

Democrats acknowledge the political value of the lawsuits. Mrs. Clinton badly needs to turn out minority voters in numbers similar to those Mr. Obama drew in his 2012 re-election.

“I think it has been a growing trend in the last 10 years for campaigns to use litigation like this as a campaign weapon,” said Hans von Spakovsky, a Republican election law expert at the conservative Heritage Foundation. “The claims that they keep making, that this is going to depress turnout, just keep proving to be not true, and many of these issues have already been litigated.”

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According to the nonpartisan Brennan Center for Justice at New York University School of Law, there are 14 states with new voter laws in place for the first time in a national race.

The lawsuits filed in Ohio and Wisconsin, like the 2014 North Carolina case, were brought by lawyers including Marc Elias, a leading Democratic lawyer on voter protection issues who represents four of the party’s national campaign committees.

Mr. Elias is also the general counsel for Mrs. Clinton’s campaign, and her aides have spoken favorably of the lawsuits.

The lawsuits are typically filed on behalf of people who say they have been, or could be, disenfranchised. In the Ohio case, a black pastor is among the plaintiffs.

Mrs. Clinton’s campaign is not a party to the lawsuits, although her aides have said her team supports them. People involved in the actions have refused to say who is paying for them.

But Mr. Elias suggested that the litigation was simply common sense. “We should all want to ensure that all eligible voters can exercise their right to vote and have their vote counted,” he said. “It’s unfortunate that some Republicans see a benefit in making it harder for people to vote.”

Since the election fight in 2000 between George W. Bush and Al Gore, court battles have become a routine and increasingly strategic component of presidential campaigns. The new round of lawsuits is occurring relatively early and is a more preventive effort than in past election cycles, experts say.

More broadly, the legal campaign is shaping up as something of a delayed Democratic response to the Republican takeovers of a number of governorships and legislatures in the same states after the rise of the Tea Party movement in 2009 and 2010, as well as to the Supreme Court’s ruling on the Voting Rights Act in 2013.

As a result of both changes, a series of stringent laws were enacted in some of those states that Democrats say are intended to curtail voting by important segments of Mr. Obama’s political coalition, African-American and younger voters in particular.

Donna Brazile, the campaign manager for Mr. Gore in the 2000 presidential race and a member of a Democratic National Committee task force that was formed after the party’s disastrous losses in 2014, said she and Mr. Elias had for months been sounding an alarm within the party about voting-rights conditions in a number of states.

Instead of idling, she said, Democrats must “proactively” seek changes in states where voter protections might be changed, whether through litigation or legislation.

Republicans have used such laws for mobilization purposes of their own, denouncing voter fraud as they appeal to an increasingly conservative party base. Mr. Walker, for example, has repeatedly boasted to Republicans across the country about having enacted tougher voting laws in Wisconsin.

“Any measure that protects our democracy by making it easier to vote and harder to cheat is a step in the right direction,” said Kirsten Kukowski, a spokeswoman for Mr. Walker. “This is a bipartisan issue, and Hillary Clinton and the Democrats are on the wrong side.”

Richard L. Hasen, an election law expert at the University of California, Irvine, called the Democratic lawsuits “long shots,” saying that so-called voter protection lawsuits had often failed in the courts. “In terms of success, some of these things are a stretch,” he said.

Mr. Elias emphasized that the seeds of the Ohio and Wisconsin suits had been planted before Mrs. Clinton was a candidate. Mr. Elias and Robby Mook, now Mrs. Clinton’s campaign manager, began collaborating on voter-protection efforts last year, before it was clear that she would run or that they would be involved, people briefed on their work said.

Mr. Elias, working with other Democrats, also explored the possibility of a ballot referendum on voter rights in Ohio this year, but decided against it, according to three people privy to those discussions but not authorized to speak publicly about them.

But their efforts now, whether parallel or in concert, involve much higher stakes. Unlike Mr. Obama, who could rely on a much wealthier Democratic National Committee to pursue voter registration for several years leading to his re-election campaign, the party’s standard-bearer this time will have no such backup: The national committee has struggled financially of late and has largely outsourced its registration efforts to nonprofits.

That means Mrs. Clinton, if she is nominated, and a string of Senate candidates may have to compensate for underwhelming registration efforts by maximizing the ability of existing Democratic voters to cast ballots.

As Mrs. Clinton prepares to head to Texas Southern University, where she will receive an award named for Barbara Jordan, a former congresswoman and civil rights leader, some Democrats see voter protections as a safe topic for her to explore at this stage of the campaign, one that will not turn off voters she needs in a general election.

But others point out that a majority of voters, in opinion polls, approve in general terms of requiring voters to present photo identification. Acceptable forms in most of the states with such laws include government-issued ID cards, passports, driver’s licenses, and employee and student cards.

Michael Waldman, the president of the Brennan Center and a former speechwriter for President Bill Clinton, said the subject was riskier than it might seem.

“There’s been such a hotly contested wave of new laws that it’s not such a safe issue anymore,” he said. “It’s not just something for Fourth of July orations.”