G.O.P. lawmakers in at least eight states controlled by the party are trying to gain broad influence over the mechanics of voting, in an effort that could further undermine the country’s democratic norms.
Democrats began pushing on Wednesday for the most substantial expansion of voting rights in a half-century, laying the groundwork in the Senate for what would be a fundamental change to the ways voters get to the polls and elections are run.
At a contentious hearing on Capitol Hill, Democratic leaders made a passionate case for a bill that would mandate automatic voter registration nationwide, expand early and mail-in voting, end gerrymandering that skews congressional districts for maximum partisan advantage and curb the influence of money in politics.
The effort is taking shape as Republicans have introduced more than 250 bills to restrict voting in 43 states and have continued to spread false accusations of fraud and impropriety in the 2020 election. It comes just months after those claims, spread by President Donald J. Trump as he sought to cling to power, fueled a deadly riot at the Capitol on Jan. 6 that showed how deeply his party had come to believe in the myth of a stolen election.
Republicans were unapologetic in their opposition to the measure, with some openly arguing that if Democrats succeeded in making it easier for Americans to vote and in enacting the other changes in the bill, it would most likely place their party permanently in the minority.
“Any American who thinks that the fight for a full and fair democracy is over is sadly and sorely mistaken,” said Senator Chuck Schumer, Democrat of New York and the majority leader. “Today, in the 21st century, there is a concerted, nationwide effort to limit the rights of citizens to vote and to truly have a voice in their own government.”
Mr. Schumer’s rare appearance at a committee meeting underscored the stakes, not just for the election process but for his party’s own political future. He called the proposed voting rollbacks in dozens of states — including Georgia, Iowa and Arizona — an “existential threat to our democracy” reminiscent of the Jim Crow segregationist laws of the past.