HOUSTON » Hillary Rodham Clinton accused potential Republican presidential rivals such as Jeb Bush of Florida and Scott Walker of Wisconsin on Thursday of trying to make it more difficult for millions of Americans to vote, laying down an early marker on voting rights in her Democratic presidential campaign.

Clinton, in one of her most partisan speeches as a presidential candidate, directly criticized Walker, Bush and two other Republican presidential hopefuls, New Jersey Gov. Chris Christie and former Texas Gov. Rick Perry.

She described those current or former governors as members of a GOP vanguard that has made it more difficult for students to vote, cut the numbers of days set aside for early voting and demanded voter ID provisions.

"Today Republicans are systematically and deliberately trying to stop millions of American citizens from voting," Clinton said at historically black Texas Southern University. "What part of democracy are they afraid of? I believe every citizen has the right to vote and I believe we should do everything we can to make it easier for every citizen to vote."

The issue is closely watched by black voters, who supported President Barack Obama by sweeping margins in 2008 and 2012 and will be an important constituency for Clinton to mobilize in next year’s election. She received an award named after Barbara Jordan, the late Texas congresswoman and civil rights leader, and the event came a little more than a week before Clinton is scheduled to deliver a major speech in New York that aides are billing as a formal campaign kickoff.

Directly challenging Republicans by name, which Clinton has largely avoided, she plunged into a partisan debate over voting rights that has roiled statehouses across the country. Democrats contend restricting voter access and registration purposely aims to suppress turnout among minority and low-income voters. Republicans say the voting changes are crucial to guard against voter fraud.

Under Walker, for example, Wisconsin requires proof of residency except for overseas and military voters. The state shortened the early voting period and increased residency requirements.

In a statement, Walker responded to the criticism leveled by the Democratic candidate: "Hillary Clinton’s rejection of efforts to make it easier to vote and harder to cheat not only defies logic, but the will of the majority of Americans. Once again, Hillary Clinton’s extreme views are far outside the mainstream."

Clinton said that in New Jersey, Christie had vetoed a bill to extend early voting. She said as Florida’s governor, Bush had conducted a "deeply flawed" purge of eligible voters, by having the names of people who were mistakenly thought to be felons removed from voting rolls.

Perry, who announced his presidential campaign earlier Thursday, approved laws in Texas that discriminated against minority voters, Clinton said.

Republicans, Clinton said, should "stop fear-mongering about a phantom epidemic of election fraud and start explaining why they’re so scared of letting citizens have their say."

Clinton said the U.S. should take dramatic steps to expand the right to vote, with universal, automatic voter registration for young people, and a new national standard of no fewer than 20 days of early, in-person voting, including weekend and evening voting.

In the home state of President Lyndon Johnson, architect of the 1965 Voting Rights Act, Clinton said the Supreme Court ruling had "eviscerated" the law, making it more likely that minority voters, the elderly and others would face consequences.

Democrats have signaled plans for a large-scale legal fight against new voter ID laws and efforts to curtail voting access. Party attorneys recently filed legal challenges to voting changes made by GOP lawmakers in the presidential battleground states of Ohio and Wisconsin.

One of the attorneys involved in the lawsuits is Marc Elias, a top elections lawyer for Democrats who is also serving as the Clinton campaign’s general counsel. The campaign is not officially involved in the lawsuits.