It wasn’t the high taxes in Nassau County, or the recent changes to New York’s bail laws that drove Lizette Sonsini, a former Democrat, to vote Republican this year.

Her reasons were more overarching.

“I don’t like the president, and the Democrats are spending too much money on things like infrastructure, when really we need politicians who are going to bring more money back into this country,” said Ms. Sonsini, 56, of Great Neck.

“Maybe if Democrats see how we’re voting in these local elections,” she said, “they will see we’re not happy with the way things are going.”

Across the country, Democrats witnessed an intense backlash on Election Day, as the party suffered major losses in Virginia and in many suburban communities like Nassau County, where Democratic leaders were swept from office by Republicans — even though registered Democrats outnumber Republicans by 100,000.

The Democratic county executive, Laura Curran, trailed her Republican opponent, Bruce Blakeman, by more than four percentage points; Mr. Blakeman has declared victory, but Ms. Curran has not conceded.

The race for district attorney, a post that has been held by a Democrat since 2006, was won by the Republican Anne Donnelly, a 32-year veteran of the district attorney’s office with little prior political experience. She coasted to a 20-point win over Todd Kaminsky, a Democratic state senator and former federal prosecutor. And the race to replace the outgoing Democratic county comptroller went to a Republican, Elaine Phillips.

Off-year elections are often hard for the party of the sitting president, but the results defied candidate expectations and bolstered arguments that President Biden’s unpopularity and the Democratic Party’s internecine battles were undermining its viability in the suburbs.

“It’s almost like we’re back temporarily to the ’60s and ’70s,” said Lawrence Levy, executive dean of the National Center for Suburban Studies at Hofstra University, referring to a time when Republicans ruled the Nassau County roost. “The real question is how long this will last.”

Four years ago, Democratic voters in Nassau County treated the 2017 election as an early referendum on President Donald J. Trump. They staged postcard-writing campaigns and held living-room fund-raisers, and an energized electorate pushed Ms. Curran to become only the third Democrat in 80 years to be county executive in Nassau.

This year, the roles were reversed: The county has more than a million registered voters; 264,000 showed up and they voted overwhelmingly Republican, seemingly ousting Ms. Curran after one term.

“There was a wave, there’s no doubt about it, even for an unapologetically pro-business, pro-public safety Democrat,” Ms. Curran said in an interview, referring to herself.

In conversations with more than a dozen Nassau County voters this week, they cited their overall disapproval of the president, their distaste for vaccine mandates and a fear of funds being diverted from the police as factors in their decision to vote Republican. Concerns over Mr. Biden’s handling of Israel also arose several times.

Among those voting Republican was Audrey Alleva, a 64-year-old Garden City resident with family in the military, who cited the president’s performance as a factor in her decision.

“I don’t like the way President Biden handled the country leaving Afghanistan,” Ms. Alleva said.

Sam Liviem, a 70-year-old Great Neck resident, cited other recent Democratic pushes as reason to cast his ballot for Republicans.

“When liberals try to push ‘defund the police,’ when they try to take down statues of people from the past, when they want to wipe out history, you are going back to the law of the jungle,” Mr. Liviem said.

Nassau County was recently ranked the safest county in the United States by U.S. News and World Report. But the Nassau Republican Party exploited fears about crime to drive voters to the polls, particularly in the case of Mr. Kaminsky, who supported changes in state bail laws that Republicans blame for the county’s recent rise in shootings, which have increased across the country during the pandemic.

In 2019, New York State curtailed bail for many nonviolent defendants, who might otherwise have stayed in pretrial detention because they could not pay. But law enforcement authorities argued the law was overly broad and faulted it for not granting judges more discretion to detain defendants they considered a risk to public safety.

Mr. Kaminsky supported the original bail reform bill. And, in a video of the 2019 Senate proceedings widely circulated by the Donnelly campaign, the senate deputy majority leader, Michael Gianaris, explicitly thanks four senators, including Mr. Kaminsky, for their support. That vote came to haunt Mr. Kaminsky during his campaign.

Though Mr. Kaminsky vastly outspent Ms. Donnelly on advertising that tried to portray him as a tough-on-crime former prosecutor — her campaign spent more than $800,000 on television and online ads, according to the state Board of Elections, while his spent about $1.3 million as of mid-October — the Donnelly campaign’s message stuck.

In one ad, the Donnelly campaign recruited the mother of a shooting victim from Syracuse. “Senator Todd Kaminsky helped write the law that set my daughter’s killer free,” says the mother, Jennifer Payne, who also appeared in a 2020 ad for Representative John Katko, a Republican from central New York.

In another Donnelly ad, viewers were met by ominous music and the mustachioed visage of John Wighaus, the president of the Nassau County Detectives Association, who held Mr. Kaminsky responsible for the release of “killers, rapists and violent thugs.”

“I think crime was on everybody’s mind, I think bail reform was on everyone’s mind,” Ms. Donnelly said in an interview. She noted that concerns about crime in New York City, which bolstered the election of Eric Adams as mayor, played a role in Nassau.

“It’s a regional issue,” Ms. Donnelly said. “It’s a countrywide issue.”

Ms. Donnelly will be the county’s first Republican district attorney originally elected as a Republican since William Cahn in the 1960s, said Joseph Cairo, the county Republican chairman. (Denis Dillon, who served as Nassau County district attorney for three decades, was elected as a Democrat before switching to the Republican Party in the 1980s.)

Ms. Curran argued anxiety about criminal justice issues seeped into her race, too.

“This bail reform issue was very motivating to voters,” said Ms. Curran, who tried to distance herself from the bail legislation by appearing on “Fox and Friends” to decry the new law as an overreach.

If state and national political issues inflamed the debate in Nassau County, local issues proved potent, too.

Ms. Curran’s opponent, Mr. Blakeman, focused his campaign almost exclusively on county taxes, even incorporating “cut Nassau taxes” into the URL for his campaign website.

For eight years before Ms. Curran’s ascension to county executive in 2018, her predecessor had frozen property assessments for tax purposes. Upon taking office, Ms. Curran began a process to rework a tax system she called “corrupt.” But after the reassessments, 65 percent of Nassau County homeowners saw higher taxes, according to a report in Newsday, and Ms. Curran’s opponents seized on anger about those tax increases.

“There’s some terrible news coming in the mail: It’s your school tax bill,” Mr. Blakeman said in one ad. “Laura Curran’s reassessment has caused property taxes to soar two years in a row. Curran’s tax hikes will continue for three more years, unless we stop her.”

“A lot of people, their school tax bill went up by thousands and thousands of dollars, and I think that was a major issue,” Mr. Blakeman said in an interview.

Even in the Democratic bastion of North Hempstead, which has in recent years seen an influx of more politically conservative Iranian Jews, the Republican candidate for a seat on the County Legislature, Mazi Melesa Pilip, was leading the Democratic incumbent, Ellen Birnbaum, by more than 15 points. The supervisor of the town of North Hempstead may soon be a Republican, too.

All that being said, Democrats continued to take refuge in the notion that they were part of something bigger than themselves, a Republican tide of unusually sweeping proportions.

“What happened in Nassau County happened in lots of places — Suffolk County, upstate New York, Virginia, New Jersey,” said Jay Jacobs, the chair of the Nassau County and state Democrats. “What happened was Republican voters overperformed dramatically and Democratic voters underperformed.”

But Jovanni Ortiz, the founder of the South Shore Democrats, laid just as much blame at the feet of Mr. Jacobs.

Mr. Ortiz argued that Mr. Jacobs failed to galvanize voters — particularly in Nassau County communities of color, in part by failing to get out the vote on Democratic ballot questions that would have made it easier for New Yorkers to vote. He has called on Mr. Jacobs to resign.

“He’s kind of putting it on national issues, which I agree are part of the reason for some of the results,” Mr. Ortiz said. “But for him to not recognize that part of this is on him is a little bit crazy.”

Chantee Lans contributed reporting.