Less than 60 days until Election Day, Democratic presidential nominee Hillary Clinton is scrambling to explain why she placed a big chunk of the electorate into a “basket of deplorables,” an unforced error that can be political kryptonite in an era of viral video and Twitter takes.

Mrs. Clinton later said she regretted her remark that half of Donald Trump’s supporters held racist, xenophobic or other “irredeemable” qualities.

Republicans, buoyed by tightening poll numbers, said the slip will haunt Democrats down the homestretch.

“I’ve got to tell you, Hillary’s 50 percent comment, it is resonating. People heard what she said,” Rep. Marsha Blackburn, Tennessee Republican, said Sunday on ABC’s “This Week.”

Indeed, the remarks drew swift comparisons to other caught-on-tape gaffes.

In 2008, Barack Obama enraged Midwesterners by clumsily referring to bitter voters who “cling to guns or religion.” Mr. Obama overcame the gaffe and rode into history, yet Republican presidential nominee Mitt Romney couldn’t recover from his “47 percent” remark four years later.

The former Massachusetts governor said almost half of Americans would vote for Mr. Obama no matter what, because they “believe that they are victims” and “believe the government has a responsibility to care for them.” Those remarks were secretly taped by a bartender at the event and seriously damaged Mr. Romney’s White House prospects.

Mr. Trump, who likes to say Mr. Romney blew it for the Republican Party, swiftly connected the dots on Twitter, where political impressions travel at light speed.

“Hillary Clinton just had her 47% moment,” Mr. Trump posted. “What a terrible thing she said about so many great Americans!”

Insults have roiled down-ballot races, too. In 2010, Sen. John F. Kerry of Massachusetts took heat for blaming Democrats’ midterm election woes on uninformed voters. “We have an electorate that doesn’t always pay that much attention to what’s going on, so people are influenced by a simple slogan rather than the facts or the truth or what’s happening,” he said.

Republicans rode a surge of tea party angst to a sweeping majority in the House that November.

Darrell West, director of governance studies at the Brookings Institution, said it is never good idea to criticize voters, “since they are the ones making the election decision.”

“Even if you dislike their stances, it is better to complain about the candidate than the voters,” he said.

Mrs. Clinton reportedly planned to steer her campaign away from personal attacks on her Republican rival. But at a New York event with lesbian, gay, bisexual and transgender (LGBT) supporters late Friday, she told donors that 50 percent of Mr. Trump’s backers were “irredeemable.”

“To just be grossly generalistic, you can put half of Trump supporters into what I call the basket of deplorables. Right? Racist, sexist, homophobic, xenophobic, Islamaphobic, you name it,” she said. “Now some of those folks, they are irredeemable, but they are not America.”

Mrs. Clinton has increasingly drawn attention to members of the nationalistic “alt-right” movement and white supremacists who are enthused by Mr. Trump’s more strident positions, including his plan to build a wall along the Mexican border and restrict immigration from places with histories of Islamist-inspired terrorism.

Yet the candidate herself realized her goof by tying a number to her claim.

“Last night, I was ‘grossly generalistic,’ and that’s never a good idea. I regret saying ‘half’ — that was wrong,” Mrs. Clinton said in a Saturday statement.

Mrs. Clinton’s remark was an attempt to explain voters’ support of Mr. Trump, which has him running neck and neck with Mrs. Clinton in some polls.

She still holds a 5-point edge over Mr. Trump, according to a Washington Post/ABC News poll of likely voters, yet she suffers from an enthusiasm gap. More than nine in 10 Trump supporters say they are certain to vote, compared with 80 percent of Mrs. Clinton’s supporters.

Meanwhile, an NBC/Wall Street Journal/Marist poll says the candidates are virtually deadlocked in a quartet of battleground states. Mr. Trump has a slight edge in red-leaning Arizona and Georgia, and Mrs. Clinton is up by 1 point in “blueish” Nevada and New Hampshire.

Mrs. Clinton can ill afford to lose working-class voters in critical swing states.
In a statement walking back her “deplorables” remarks, she stressed that not all of Mr. Trump’s supporters belonged in the same basket.

“As I said, many of Trump’s supporters are hard-working Americans who just don’t feel like the economy or our political system are working for them,” Mrs. Clinton said. “I’m determined to bring our country together and make our economy work for everyone — not just those at the top. Because we really are ‘stronger together.’”

However, Mrs. Clinton said that she stood by her criticism that the Republican nominee appealed to the worst of America.

“Let’s be clear: What’s really deplorable is that Donald Trump hired a major advocate for the so-called alt-right movement to run his campaign and that David Duke and other white supremacists see him as a champion of their values,” said Mrs. Clinton. “It’s deplorable that Trump has built his campaign largely on prejudice and paranoia and given a national platform to hateful views and voices, including by retweeting fringe bigots with a few dozen followers and spreading their message to 11 million people.”

Republicans saw it differently. Mr. Trump’s running mate, Mike Pence, said it revealed Mrs. Clinton’s disdain for the American people, and the Republican presidential nominee painted Mrs. Clinton’s attempt to walk back her comments as “disgraceful.”

⦁ Seth McLaughlin contributed to this report.