Biden blasts GOP voting restrictions push: 'The big lie is just that -- a big lie!'

President Joe Biden on Tuesday decried Republican efforts to limit ballot access across the country as a "21st century Jim Crow assault," while warning Americans that the GOP push to restrict voting and "selfish" challenge of the 2020 election results were "the most significant test of our democracy since the Civil War."

"There is an unfolding assault taking place in America today, an attempt to suppress and subvert the right to vote in fair and free elections, an assault on democracy, an assault on liberty, an assault on who we are as Americans," Biden said at the National Constitution Center in Philadelphia.

While he encouraged more Americans to participate in the political process, Biden stopped short of endorsing changes to the Senate filibuster rule that would allow Democrats to pass voting rights measures with a simple majority vote -- something advocates, and many Democrats, have begged him to support.

Calling for a coalition of Americans across party and class lines to meet "the urgency of the moment" and stand up to GOP-led efforts in 16 states to rewrite voting laws, Biden also asked Congress to take up Democrats' proposed overhaul of the federal election system, the For the People Act, which has stalled in the Senate amid GOP opposition.

He also asked for consideration of the voting rights bill named for the late Rep. John Lewis, D-Ga., that would restore elements of the Voting Rights Act of 1965 struck down by the Supreme Court in 2013.

The court's decision earlier this month to impose new limits on another section of the landmark law also "puts the burden on Congress" to "restore" the 1965 law, Biden said Tuesday.

He wasted no time taking a shot at former President Donald Trump and his supporters, homing in on the 2020 election as the "most scrutinized election in American history.

"More than 80 judges, including those appointed by my predecessor heard the arguments. In every case, neither cause nor evidence was found to undermine the national achievement of administering the historic election," he said.

"The big lie is just that -- a big lie!" he declared.

"In America, if you lose, accept the results. Follow the Constitution. Try again. You don't call facts fake and try to bring down the American experiment just because you're unhappy," he continued. "That's not statesmanship -- that's selfishness."

He called passing national voting legislation "a national imperative."

"Republicans opposed even debating, even considering the For the People Act. Senate Democrats stood united to protect our democracy and the sanctity of the vote. We must pass the For the People Act," Biden said to applause.

Biden linked the ongoing Republican efforts in state legislatures to the violence at the US Capitol on Jan. 6, when Trump-supporting rioters broke into the building and assaulting police officers to derail the Electoral College count.

"Because of the extraordinary courage of elections officials, many of them Republicans, our court system, and those brave Capitol police officers -- because of them, democracy held," Biden said. "Look how close it came.

"We're going to face another test in 2022, a new wave of voter suppression and raw and sustained election subversion. We have to prepare now," he said.

His use of the bully pulpit comes as his administration wades more aggressively into the fight over ballot access at the urging of civil rights groups and Democrats as Republican-led legislatures advance new voting restrictions in places like Texas and Congress remains deadlocked over proposed legislation.

Sixteen states have enacted 28 laws that would restrict voting access, out of hundreds that have been introduced throughout the country, according to the Brennan Center for Justice at NYU School of Law.

Biden hinted at future administration action on voting rights, saying that he and Vice President Kamala Harris "will be making it clear that there is real peril in making raw power, rather than the idea of liberty, the centerpiece of the common life."

His speech also comes as Democrats in the Texas State Legislature have fled their state for Washington, D.C., the second such effort in recent weeks in an attempt to prevent a vote on legislation they say will roll back voting rights in their state.

The state lawmakers said in a press conference earlier Tuesday outside the Capitol that they're there to pressure Congress to pass federal voting rights legislation and call for an exception to the Senate's filibuster rule blocking Democrats from moving forward with a measure, they say, would stop GOP-led efforts to restrict voting in Texas and nationwide.

In March, House Democrats advanced the For the People Act, an expansive package that would transform federal elections, voting and congressional redistricting -- but it has stalled in the Senate after failing to advance in a procedural vote late last month, over opposition from all Republicans.

In light of the GOP opposition, some Democrats have pushed for the Senate to reform the legislative filibuster, with House Democratic Whip Jim Clyburn, D-S.C., a key Biden ally and endorser during the 2020 Democratic primary, suggesting Democrats create an exception to the 60-vote threshold for election reform and other constitutional issues. Because of their opposition to ending the filibuster, Democratic Sens. Joe Manchin of West Virginia and Kyrsten Sinema of Arizona are playing a pivotal role in the ongoing congressional negotiations over a national voting rights bill.

On Monday, White House press secretary Jen Psaki rejected Clyburn's calls to alter the filibuster, saying any changes to Senate rules "will be made by members of the Senate, not by this president or any president, frankly, moving forward."

Biden did not directly mention Manchin, Sinema or the filibuster in his remarks Tuesday.

“I’m not filibustering now,” Biden told the traveling pool reporter when asked why he didn't bring up the legislative filibuster during his remarks.

He was also non-committal on the topic in his meeting with civil rights leaders last week, Rev. Al Sharpton told the pool reporter in Philadelphia.

"He said, 'We're working through'" the issue, Sharpton told reporters after Biden's event.

ABC News' Alisa Wiersema and Molly Nagle contributed to this report.