Democratic presidential candidate Hillary Clinton speaks at Texas Southern University in Houston on Thursday. (Melina Mara/The Washington Post)

Hillary Rodham Clinton on Thursday called for sweeping changes in national voter-access laws aimed at making it easier for young people and minorities to take part in elections, putting her on a collision course with Republicans who say such measures are a political ploy that would lead to widespread abuses.

In a speech at a historically black college here, Clinton called for federal legislation that would automatically register Americans to vote at age 18 and would mandate at least 20 days of early voting ahead of election days in all states.

Making her most fiercely partisan political speech since her first, failed run for president in 2008, Clinton attacked Republicans for what she characterized as a calculated attempt to turn back the clock on voting rights — and called out several potential 2016 opponents by name for backing voter restrictions as governors.

“Today Republicans are systematically and deliberately trying to stop millions of American citizens from voting,” Clinton said in a speech at Texas Southern University. “What part of democracy are they afraid of?”

The pointed attacks and extensive policy proposals signal that Clinton intends to make voter access a major plank in her campaign platform — a move aimed at firing up the Democratic base and portraying her GOP opponents as suppressing votes. Her campaign’s top lawyer, Marc Elias, has co-filed lawsuits over voting access in Ohio and Wisconsin — both key presidential battleground states with Republican governors who may join the 2016 race.

The crowd cheers Democratic presidential candidate Hillary Clinton after she spoke at Texas Southern University on Thursday. (Melina Mara/The Washington Post)

The Republican National Committee accused Clinton of being “misleading and divisive” and noted that her home state of New York does not provide early voting. “Her exploitation of this issue only underscores why voters find her dishonest and untrustworthy,” RNC spokesman Orlando Watson said in a statement.

During her speech, Clinton said Republican state legislatures are intentionally restricting voting by curtailing early access to the polls and other measures in an effort to ­suppress Democratic turnout. Among the potential opponents she singled out for criticism were New Jersey Gov. Chris Christie, Wisconsin Gov. Scott Walker, former Florida governor Jeb Bush and former Texas governor Rick Perry. Perry announced his second run for the White House on Thursday.

“Today there are people who offer themselves to be leaders whose actions have undercut this fundamental American principle” of a free vote, Clinton said.

Perry spokesman Travis Considine said Clinton’s remarks demonstrate “how truly out of touch she is with the people of Texas.”

“While it is unfortunate, Gov. Perry is not surprised that Hillary Clinton would come to Texas and call for weakening the integrity of our election process,” Considine said in a statement.

Nationwide mandatory voter registration would generally help Democrats, whose support frequently comes from younger and poorer people and minority groups that may also be less likely to sign up to vote at 18 on their own. That change and a mandatory minimum period for early voting would have to be approved by Congress — now controlled by Republicans — so it is unlikely to happen in time to benefit Clinton in the 2016 election if she is the Democratic nominee.

“None of them will come easily,” she acknowledged in her speech.

Clinton also alleged that Republican efforts to limit voter registration have a disproportionate impact on “people of color, poor people and young people from one end of our country to the other.”

Under universal voter registration, every citizen would be automatically registered to vote on their 18th birthdays, unless they actively opt out.

About 71 percent of eligible adults nationwide are registered to vote, according to census figures, and a lower percentage actually show up at the polls. Registration and turnout tend to be higher among older and relatively affluent white voters, who are also more likely to vote Republican.

The requirement for in-person early voting that Clinton seeks would also mandate that polling places have weekend and evening hours.

Although early voting has become fairly common in the past decade, many Republicans say it increases the opportunity for fraudulent voting. Republicans have raised similar objections to same-day registration and other efforts — many of them led by Democrats — to make voting easier or more convenient. Clinton dismissed such complaints as unfounded.

Election analysts generally agree that voter fraud is rare, although there have been a handful of well-publicized examples of fraudulent names being added to the rolls.

Clinton’s address comes as Democrats are pursuing legal challenges to voting rule changes approved by Republican legislatures in several states.

“This is, I think, a moment when we should be expanding the franchise,” Clinton’s campaign chairman, John Podesta, said in an interview Wednesday. “What we see in state after state is this effort by conservatives to restrict the right to vote.”

Attorneys involved in the lawsuits in Ohio and Wisconsin over voter access have similar views.

“This lawsuit concerns the most fundamental of rights guaranteed citizens in our representative democracy — the right to vote,” the attorneys wrote in a federal complaint filed Friday in Wisconsin.

Walker spokeswoman Kirsten Kukowski said voter-access restrictions make it “easier to vote and harder to cheat” and added, “This is a bipartisan issue, and Hillary Clinton and the Democrats are on the wrong side.”

Since the 2010 Republican wave, 21 states have implemented new laws restricting voting access, some cutting back on early voting hours and others limiting the number of documents considered valid identification to vote, according to a new analysis from the Brennan Center for Justice, a nonpartisan think tank at the New York University School of Law. For 14 of those states, the 2016 contest will be the first presidential election with the new restrictions in place.

Some limits also flowed from the 2013 Supreme Court decision that invalidated some parts of the Voting Rights Act of 1965. The day that decision came down, Perry praised it as a “clear victory for federalism and the states” and vowed to proceed with the implementation of a strict photo ID requirement, previously blocked under the law.

That requirement is currently being challenged in court, with a resolution expected as soon as this summer.

About three dozen states and the District offer early voting of some kind, allowing voters to cast ballots before Election Day without an excuse. The average early voting period is roughly 22 days, the bipartisan National Conference of State Legislatures reported earlier this year.

Oregon’s breakthrough “new motor voter” law passed earlier this year is the closest any state has come to the kind of automatic registration endorsed by Clinton. She praised Oregon as a leader in modernizing ­antiquated voting procedures, including paper registration.

Under the new law, all Oregonians applying for a new or updated driver’s license are automatically added to the voter rolls, unless they opt out. The state has estimated that the law will add about 300,000 voters to the rolls.

Younger voters are the least likely to be registered and have tilted toward Democrats in recent years. In 2012, the Census Bureau reported 57 percent of citizens under 30 were registered to vote, compared with 78 percent of those 55 and older. Voters under 30 supported Barack Obama by a 29 percentage-point margin over Mitt Romney, according to network exit polls (66 percent to 37 percent).

In 2008 and 2012, African American turnout rates surged to match or exceed turnout among whites for the first time, but a central question in 2016 is whether blacks will turn out at similar levels when President Obama is not on the ballot.

Hispanics and Asians — groups Obama won by wide margins as well — vote at far lower rates than whites and African Americans, representing a large untapped pool of Democratic support. Automatic registration among these groups may encourage more voting participation.

Exit polls in 2012 found that Obama racked up a seven-point lead over Republican Mitt Romney among early voters, compared with a one-point edge among those casting ballots on Election Day.

Democrats’ advantage among early voters was less clear according to voter registration data tracked by the U.S. Elections Project. In five of seven states where data are available, Democrats made up about the same percentage of early voters as they did on Election Day.

Chokshi reported from Washington. Scott Clement in Washington contributed to this report.