PHILADELPHIA (REUTERS) - President Joe Biden, under pressure from US civil rights leaders, on Tuesday (July 13) called it a "national imperative" to pass sweeping voting rights legislation that has stalled in Congress but did not outline a path to overcome Republican opposition.

Numerous Republican-controlled states have passed new voting restrictions this year, a push encouraged by Biden's Republican predecessor Donald Trump.

In a passionate speech in a city considered the nation's birthplace, Biden, without naming him, took aim at Trump for false claims that the 2020 election was stolen from the Republican former president through widespread voting fraud.

"So hear me clearly: there's an unfolding assault taking place in America today, an attempt to suppress and subvert the right to vote in fair and free elections," Biden told a cheering crowd in Philadelphia.

"The Big Lie is just that: a big lie," Biden said, referring to unfounded fraud election claims by Trump and his supporters.

The voting rights legislation faces an uphill battle in Congress, where Biden's fellow Democrats have been stymied by Senate Republicans who blocked it even from being debated.

Biden's focus on the subject, even if the legislation fails, enables him to rally support among Democratic voters as his party works to maintain control of Congress in the 2022 midterm elections.

Biden made a searing critique of what he called efforts to undermine voting rights, likening them to past laws that prevented Black people and women from voting in the United States.

"They want to make it so hard... that they hope people don't vote at all. That's what this is about," Biden said of those efforts.

"We must pass the 'For the People Act.' It's a national imperative," Biden added, referring to the legislation.

According to the Brennan Centre for Justice at New York University Law School, at least 17 states this year have enacted laws that restrict voting access, with more being considered.

Biden's party and civil rights groups have opposed these restrictions, which critics have said are aimed at Black, Hispanic and younger voters, who have helped elect Democrats.

Many Republicans have justified new restrictions as a means to combat voter fraud, a phenomenon that election experts have said is rare in the United States.

Two of the nation's foundational documents, the Declaration of Independence and the US Constitution, were signed at Independence Hall, just steps away from the National Constitution Centre where Biden made his speech.

The filibuster

Biden did not announce a new position on a Senate custom called the filibuster under which most legislation cannot advance without the support of 60 members of the 100-seat chamber. The Senate is split 50-50 between the parties, with Democrats in control because Vice-President Kamala Harris can cast a tie-breaking vote.

Some advocates want Biden to back an arrangement that would prevent the filibuster from blocking the voting rights Bill.

Biden, who served in the Senate for decades, has resisted calling for ending the filibuster.

Civil rights leader Al Sharpton noted to Reuters after the speech that Biden did not mention the filibuster, and said he had just spoken to the president. "And he said to me just now,'Al, we're still working through where we are going to be on that.' He's not committed yet."

If passed, the Democratic-backed Bill would expand opportunities to cast ballots before Election Day, make certain campaign contributions more transparent and change the process for drawing the boundaries of House of Representatives districts. Republicans said the measure violates the authority of states to set their own election laws.

"After Democrats failed to pass their federal takeover of our elections... Biden is continuing their dishonest attacks on commonsense election integrity efforts," Republican National Committee spokesman Danielle Alvarez said.

"Meanwhile, Republicans are engaged in state-led efforts to make it easier to vote and harder to cheat, and polling shows Americans overwhelmingly support these laws."

Biden met last week with civil rights leaders who prodded him to keep fighting for voting rights despite Republican resistance.

In Texas on Monday, more than 50 Democratic legislators left that state in a bid to derail Republican efforts to pass voting restrictions. The Democratic legislators said on Tuesday they plan to stay in Washington as long as needed to derail the state legislation and push for federal voting reform.

US Representative Lloyd Doggett, who appeared with the Texas lawmakers at a news conference in Washington, recalled how difficult it was for the landmark Voting Rights Act of 1965 to pass under Democratic President Lyndon Johnson.

"We need the president and the vice president, and every Democrat in this Senate, working together to preserve American democracy," Doggett said.

Harris lauded the Texas legislators.

"I think they have shown great courage, and certainly great conviction and commitment," Harris said in an interview with Reuters.