Obama's administration has challenged states that have implemented voter ID laws and other restrictions in the wake of a Supreme Court decision that struck down part of the 1965 Voting Rights Act, designed to prevent discrimination at the polls.
Strict voting rights laws are said to disproportionately affect minorities and lower-income Americans, many of whom tend to vote for Democratic candidates.
"The stark, simple truth is this: The right to vote is threatened today in a way that it has not been since the Voting Rights Act became law nearly five decades ago," Obama told a meeting of the National Action Network, a group founded by civil rights leader and MSNBC television anchor Reverend Al Sharpton.
"Across the country, Republicans have led efforts to pass laws making it harder, not easier, for people to vote," he said.
Last year the Obama administration sued North Carolina to block rules including a requirement for voters to show photo identification at the polls. The Justice Department also sued to keep Texas from carrying out a voter identification requirement enacted in 2011.
Proponents of such rules argue they are needed to prevent voter fraud.
North Carolina Republican Governor Pat McCrory signed the state's sweeping voting changes into law in August, saying: "Common practices like boarding an airplane and purchasing Sudafed require photo ID, and we should expect nothing less for the protection of our right to vote."
Obama said studies showed abuse was extremely rare.
"The real voter fraud is people who try to deny our rights by making bogus arguments about voter fraud," he said to applause from the crowd.
"But it's a fact this recent effort to restrict the vote has not been led by both parties. It's been led by the Republican Party," he continued. "If your strategy depends on having fewer people show up to vote, that's not a sign of strength. That's a sign of weakness."
The president has been targeting Republicans on issues that are important to Democrats ahead of congressional elections in November. The Republican National Committee said his Friday speech was all about politics.
"President Obama was in campaign mode today with a speech designed to mobilize his base," said Orlando Watson, an RNC spokesman. "The president made clear that the biggest problem for Democrats in 2014 is voter enthusiasm not voter access."
Obama flew to Texas earlier this week to commemorate the 50th anniversary of the passage of the Civil Rights Act of 1964, the landmark legislation spearheaded by President Lyndon B. Johnson that helped end America's segregationist past.
Obama praised Johnson for pushing the Voting Rights Act despite counsel from some of his advisers to wait.
"Johnson said, 'About this there can and should be no argument. Every American citizen must have an equal right to vote," Obama said, quoting the former president.
"The principle of one person, one vote is the single greatest tool we have to redress an unjust status quo. You would think there would not be an argument about this anymore."