Donna Brazile, a CNN contributor and a Democratic strategist, is vice chairwoman for voter registration and participation at the Democratic National Committee. A nationally syndicated columnist, she is an adjunct professor at Georgetown University and author of "Cooking With Grease: Stirring the Pots in America." The opinions expressed in this commentary are solely those of the author.

(CNN) Hillary Clinton is speaking Thursday in Texas about the importance of protecting and expanding the constitutional right to vote for all citizens of the United States. Clinton will also call for a new national standard of "no fewer than 20 days of early in-person voting, including weekend and evening voting."

It has been 50 years since the historic march led by John Lewis, the Rev. Martin Luther King Jr. and civil rights activists from Selma, Alabama, to Montgomery, Alabama, protesting voting restrictions used against African-Americans who wanted to exercise their right to vote.

It was only a few months later, on August 6, 1965, when President Lyndon Johnson signed into law the Voting Rights Act to end all barriers to political participation for racial and ethnic minorities, once and for all.

While a great deal of progress has been made over the years, today, we face new challenges to those who are eligible to vote, and some of the leading culprits of erecting barriers are running for president.

It shouldn't come as a surprise to anyone that Republicans are imposing new restrictions on the right to vote -- restrictions that disproportionally affect African-Americans, Latinos, working Americans, seniors and America's youth -- the very groups the Voting Rights Act was formed to protect.

It is because of these restrictions that we must continue to march for the right of all eligible citizens to exercise their right to vote. We march because the right to have one's voice heard in this country is embodied in our Constitution, and in the spirit of our democracy.

That's the fundamental difference between Democrats and the current GOP presidential field. As Democrats, we believe in the right of every eligible citizen to vote and have that vote counted; we are fighting to expand and protect the right to vote, while Republicans are doing just the opposite.

Many of the Republican presidential hopefuls would make it more difficult for people to cast their ballots because they believe that is their party's best path to victory, to have a smaller electorate with fewer minority voters.

While a small number of Republicans have worked to restore protections to voting rights struck down by the Supreme Court, too many of them support laws that prevent people who disagree with them from participating in the electoral process.

Jeb Bush has a long history of opposing basic voting rights, calling for more stringent voter ID laws and signing into law legislation that restricted the hours and locations for early voting. And let's not forget when the Jeb Bush administration purged 12,000 eligible voters from the Florida voter rolls ahead of the 2000 presidential election, and attempted a similar voter purge ahead of the 2004 election.

Republican hopeful, Sen. Marco Rubio also supported voter ID laws. While on the campaign trail for Mitt Romney, Rubio stated his support for these laws, saying "What's the big deal? What is the big deal?" Well according to the Brennan Center for Justice, the "big deal" is that more than 11% of eligible voters lack government-issued identification.

Former Texas Gov. Rick Perry also signed legislation to make Texas voter ID laws stricter. Even though Texans were already required to bring an ID with them to the polls, the new law would have required them to bring a photo ID. The Department of Justice blocked the law, stating that it violated the Voting Rights Act and disproportionately affected Hispanic voters.

Sen. Ted Cruz , son of a Cuban immigrant, introduced federal legislation that would have required voters to present "proof of citizenship" before casting their ballots in elections for federal office. This measure would have amended the National Voter Registration Act.

Sen. Rand Paul claims he is the single GOP candidate qualified to broaden his party's appeal to constituencies they normally ignore. But when asked about the need to strengthen the Voting Rights Act, Paul suggested it wasn't necessary because "we have an African-American president."

Ohio Gov. John Kasich signed into law a piece of legislation that slashed early voting and ended same-day registration in the state. The Cleveland Plain Dealer's editorial board even said the legislation was "a breathtaking bid to suppress voting despite constitutional guarantees of voting rights."

And then there's Wisconsin Gov. Scott Walker, who has arguably the worst record among the GOP's potential candidates in terms of voting rights. In 2010, Walker enacted one of the most restrictive voter ID laws in the country that also cut the early voting period from 30 to 12 days, eliminated weekend and evening voting, and required proof of residence when registering to vote, along with many other strict provisions.

To no one's surprise, lawsuits were filed in recent weeks maintaining that Ohio and Wisconsin laws violated the Voting Rights Act. In Ohio, the lawsuit claimed that "hundreds of thousands of Ohioans will find it substantially more difficult to exercise" their right to vote.

In Wisconsin, the lawsuit was specifically filed against the voting laws passed under the Walker administration, claiming that the measures targeted "African-American, Latino, young, and/or Democratic voters in Wisconsin in particular."

Clearly, these guys just don't get it.

It's time the GOP presidential candidates get behind one of their party's own elected officials, Rep. Jim Sensenbrenner , and others who want to fulfill the promise of the Constitution and protect everyone's right to vote.

Listen, at the end of the day, the right to vote is sacred in this country, especially for the people who have fought so hard over the years to have their voices heard. When Republican lawmakers play politics and take that away, they silence the voices of so many across the country. I'm glad Clinton is speaking out on this issue and, who knows, we might get a chance to hear from others before the end of the summer.