In the days after the Jan. 6 attack on the Capitol, the phone lines and websites of local election officials across the country were jumping: Tens of thousands of Republicans were calling or logging on to switch their party affiliations.

In California, more than 33,000 registered Republicans left the party during the three weeks after the Washington riot. In Pennsylvania, more than 12,000 voters left the G.O.P. in the past month, and more than 10,000 Republicans changed their registration in Arizona.

An analysis of January voting records by The New York Times found that nearly 140,000 Republicans had quit the party in 25 states that had readily available data (19 states do not have registration by party). Voting experts said the data indicated a stronger-than-usual flight from a political party after a presidential election, as well as the potential start of a damaging period for G.O.P. registrations as voters recoil from the Capitol violence and its fallout.

Among those who recently left the party are Juan Nunez, 56, an Army veteran in Mechanicsburg, Pa. He said he had long felt that the difference between the United States and many other countries was that campaign-season fighting ended on Election Day, when all sides would peacefully accept the result. The Jan. 6 riot changed that, he said.

“What happened in D.C. that day, it broke my heart,” said Mr. Nunez, a lifelong Republican who is preparing to register as an independent. “It shook me to the core.”

The biggest spikes in Republicans leaving the party came in the days after Jan. 6, especially in California, where there were 1,020 Republican changes on Jan. 5 — and then 3,243 on Jan. 7. In Arizona, there were 233 Republican changes in the first five days of January, and 3,317 in the next week. Most of the Republicans in these states and others switched to unaffiliated status.

Voter rolls often change after presidential elections, when registrations sometimes shift toward the winner’s party or people update their old affiliations to correspond to their current party preferences, often at a department of motor vehicles. Other states remove inactive voters, deceased voters or those who moved out of state from all parties, and lump those people together with voters who changed their own registrations. Of the 25 states surveyed by The Times, Nevada, Kansas, Utah and Oklahoma had combined such voter list maintenance with registration changes, so their overall totals would not be limited to changes that voters made themselves. Other states may have done so, as well, but did not indicate in their public data.

Among Democrats, 79,000 have left the party since early January.

But the tumult at the Capitol, and the historic unpopularity of former President Donald J. Trump, have made for an intensely fluid period in American politics. Many Republicans denounced the pro-Trump forces that rioted on Jan. 6, and 10 Republican House members voted to impeach Mr. Trump. Sizable numbers of Republicans now say they support key elements of President Biden’s stimulus package; typically, the opposing party is wary if not hostile toward the major policy priorities of a new president.

“Since this is such a highly unusual activity, it probably is indicative of a larger undercurrent that’s happening, where there are other people who are likewise thinking that they no longer feel like they’re part of the Republican Party, but they just haven’t contacted election officials to tell them that they might change their party registration,” said Michael P. McDonald, a professor of political science at the University of Florida. “So this is probably a tip of an iceberg.”

But, he cautioned, it could also be the vocal “never Trump” reality simply coming into focus as Republicans finally took the step of changing their registration, even though they hadn’t supported the president and his party since 2016.

Kevin Madden, a former Republican operative who worked on Mitt Romney’s 2012 presidential campaign, fits this trend line, though he was ahead of the recent exodus. He said he changed his registration to independent a year ago, after watching what he called the harassment of career foreign service officials at Mr. Trump’s first impeachment trial.

“It’s not a birthright and it’s not a religion,” Mr. Madden said of party affiliation. “Political parties should be more like your local condo association. If the condo association starts to act in a way that’s inconsistent with your beliefs, you move.”

As for the overall trend of Republicans abandoning their party, he said that it was too soon to say if it spelled trouble in the long term, but that the numbers couldn’t be overlooked. “In all the time I worked in politics,” he said, “the thing that always worried me was not the position but the trend line.”

Some G.O.P. officials noted the significant gains in registration that Republicans have seen recently, including before the 2020 election, and noted that the party had rebounded quickly in the past.

“You never want to lose registrations at any point, and clearly the January scene at the Capitol exacerbated already considerable issues Republicans are having with the center of the electorate,” said Josh Holmes, a top political adviser to Senator Mitch McConnell, the minority leader. “Today’s receding support really pales in comparison to the challenges of a decade ago, however, when Republicans went from absolute irrelevance to a House majority within 18 months.”

He added, “If Republicans can reunite behind basic conservative principles and stand up to the liberal overreach of the Biden administration, things will change a lot quicker than people think.”

In North Carolina, the shift was immediately noticeable. The state experienced a notable surge in Republicans changing their party affiliation: 3,007 in the first week after the riot, 2,850 the next week and 2,120 the week after that. A consistent 650 or so Democrats changed their party affiliation each week.

But state G.O.P. officials downplayed any significance in the changes, and expressed confidence that North Carolina, a battleground state that has leaned Republican recently, will remain in their column.

“Relatively small swings in the voter registration over a short period of time in North Carolina’s pool of over seven million registered voters are not particularly concerning,” Tim Wigginton, the communications director for the state party, said in a statement, predicting that North Carolina would continue to vote Republican at the statewide level.

In Arizona, 10,174 Republicans have changed their party registration since the attack as the state party has shifted ever further to the right, as reflected by its decision to censure three Republicans — Gov. Doug Ducey, former Senator Jeff Flake and Cindy McCain — for various acts deemed disloyal to Mr. Trump. The party continues to raise questions about the 2020 election, and last week Republicans in the State Legislature backed arresting elections officials from Maricopa County for refusing to comply with wide-ranging subpoenas for election equipment and materials.

It is those actions, some Republican strategists in Arizona argue, that prompted the drop in G.O.P. voter registrations in the state.

“The exodus that’s happening right now, based on my instincts and all the people who are calling me out here, is that they’re leaving as a result of the acts of sedition that took place and the continued questioning of the Arizona vote,” said Chuck Coughlin, a Republican strategist in Arizona.

For Heidi Ushinski, 41, the decision to leave the Arizona Republican Party was easy. After the election, she said, she registered as a Democrat because “the Arizona G.O.P. has just lost its mind” and wouldn’t “let go of this fraudulent election stuff.”

“The G.O.P. used to stand for what we felt were morals, just character, and integrity,” she added. “I think that the outspoken G.O.P. coming out of Arizona has lost that.”

This is the third time Ms. Ushinski has switched her party registration. She usually re-registers to be able to vote against candidates. This time around, she did it because she did not feel that there was a place for people like her in the “new” Republican Party.

“I look up to the Jeffry Flakes and the Cindy McCains,” she said. “To see the G.O.P. go after them, specifically, when they speak in ways that I resonate with just shows me that there’s nothing left in the G.O.P. for me to stand for. And it’s really sad.”

Mr. Nunez, the Army veteran in Pennsylvania, said his disgust with the Capitol riot was compounded when Republicans in Congress continued to push back on sending stimulus checks and staunchly opposed raising the minimum wage to $15 an hour.

“They were so quick to bail out corporations, giving big companies money, but continue to fight over giving money to people in need,” said Mr. Nunez, who plans to change parties this week. “Also, I’m a business owner and I cannot imagine living on $7 an hour. We have to be fair.”

Though the volume of voters leaving the G.O.P. varied from state to state, nearly every state surveyed showed a noticeable increase. In Colorado, roughly 4,700 Republican voters changed their registration status in the nine days after the riot. In New Hampshire, about 10,000 left the party’s voter rolls in the past month, and in Louisiana around 5,500 did as well.

Even in states with no voter registration by party, some Republicans have been vocal about leaving.

In Michigan, Mayor Michael Taylor of Sterling Heights, the fourth-largest city in the state, already had one foot out the Republican Party door before the 2020 elections. Even as a lifelong Republican, he couldn’t bring himself to vote for Mr. Trump for president after backing him in 2016. He instead cast a ballot for Mr. Biden.

After the election, the relentless promotion of conspiracy theories by G.O.P. leaders, and the attack at the Capitol, pushed him all the way out of the party.

“There was enough before the election to swear off the G.O.P., but the incredible events since have made it clear to me that I don’t fit into this party,” Mr. Taylor said. “It wasn’t just complaining about election fraud anymore. They have taken control of the Capitol at the behest of the president of the United States. And if there was a clear break with the party in my mind, that was it.”

Mr. Taylor plans to run for re-election this year, and even though it’s a nonpartisan race, community members are well aware of the shift in his thinking since the last citywide election in 2017.

He already has two challengers, including a staunch Trump supporter, who has begun criticizing Mr. Taylor for his lack of support for the former president.