President Obama deplored on Friday what he called a Republican campaign to deny voting rights to millions of Americans as he stepped up efforts to rally his political base heading into a competitive midterm campaign season.

Appearing at the annual convention of the Rev. Al Sharpton’s National Action Network in Manhattan, Mr. Obama accused Republicans of trying to rig the elections by making it harder for older people, women, minorities and the impoverished to cast ballots in swing states that could determine control of the Senate.

“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,” Mr. Obama said in a hotel ballroom filled with cheering supporters, most of them African-American. “Across the country, Republicans have led efforts to pass laws making it harder, not easier, for people to vote.”

Speaking a day after a conference in Texas commemorating the 50th anniversary of the Civil Rights Act, Mr. Obama linked the issue to the movement that helped pave the way for him to become the nation’s first black president.