The first openly gay major presidential candidate, Mr. Buttigieg rose to the primary’s top tier, but was unable to build a broad coalition of voters, and lost badly in South Carolina Saturday.

SOUTH BEND, Ind. — Pete Buttigieg, the former small-city Indiana mayor and first openly gay major presidential candidate, said Sunday night he was dropping out of the Democratic race, following a crushing loss in the South Carolina primary where his poor performance with black Democrats signaled an inability to build a broad coalition of voters.

The decision comes just 48 hours before the biggest voting day of the primary, Super Tuesday, when 15 states and territories will allot a third of the delegates over all. The results were widely expected to show him far behind Joseph R. Biden Jr. and Bernie Sanders.

Mr. Buttigieg canceled plans for a Sunday night rally in Dallas and a Monday morning fund-raiser in Austin, Tex., to return to South Bend, Ind., to address supporters. “Sometimes the longest way around really is the shortest way home,’’ he said.

“The truth is that the path has narrowed to a close, for our candidacy if not for our cause,” he said, adding “Tonight I am making the difficult decision to suspend my campaign for the presidency.”

Invoking his experience as a two-term mayor of South Bend, he said his candidacy caught on not in spite of that experience but because of it. America, he said, was “eager to get Washington to start running like our best run communities.”

On a conference call with campaign donors on Sunday evening, Mr. Buttigieg said he had reached the decision with regret but concluded it was “the right thing to do, when we looked at the math,” according to one person on the call. Without mentioning opponents by name, Mr. Buttigieg said he was concerned about the impact he would have on the race by staying in, saying Democrats needed to field “the right kind of nominee” against Mr. Trump.It is unclear if Mr. Buttigieg will endorse another candidate. He and Mr. Biden have exchanged voice messages, a Biden campaign official said.Mr. Buttigieg’s departure was another step in the narrowing of a Democratic field that once featured two dozen candidates, and now has six. His move comes one day after Tom Steyer, the billionaire former hedge fund executive, dropped out after a disappointing finish in South Carolina, where he invested millions of dollars.

Mr. Buttigieg, 38, skyrocketed from obscurity into the top tier of a field of more than two dozen Democratic presidential candidates largely on the strength of his robust fund-raising totals early last year. He collected more than $24 million in the three-month period ending June 30, more than any other candidate in the field.

The campaign spent nearly all of its funds to deliver its virtual tie for first place in Iowa and a narrow second-place finish behind Mr. Sanders in New Hampshire. But the rush of contributions the campaign expected after Iowa and New Hampshire never materialized. The Iowa Democratic Party’s vote-counting fiasco robbed Mr. Buttigieg of some of the expected momentum and media attention after the state’s caucuses, and Senator Amy Klobuchar was the big story after her surprise third-place showing in New Hampshire.

And Mr. Buttigieg never broadened his breadth of support in a party with a large component of nonwhite voters, and one that has veered leftward since 2018.

He came in a distant third in the Nevada caucuses, which drew strong numbers of Latino voters, and then fourth place in South Carolina, where black voters made up a majority of the Democratic electorate. He won just 3 percent of them, according to exit polls.After raising more than $76 million in 2019, an astonishing haul for a mayor with no national profile, Mr. Buttigieg spent nearly all his treasure in Iowa and New Hampshire. He faced campaigning coast-to-coast for Super Tuesday with evaporating funds and little chance of clearing the threshold of 15 percent of votes needed to amass delegates.In the last presidential debate, on Tuesday in South Carolina, Mr. Buttigieg forcefully warned that nominating Mr. Sanders, the front-runner, would lead to crushing defeat in the fall, not just “four more years of Donald Trump,” but the loss of the Democratic House majority secured by moderate candidates who won in suburban swing districts in 2018.

But Mr. Buttigieg’s own existential crisis was his inability to appeal to voters of color, both African Americans and Latinos.

Many establishment Democratic officials have openly worried about the party’s moderate candidates cannibalizing the center-left vote and making it impossible to coalesce and challenge Mr. Sanders.

Mr. Buttigieg on Monday said in a town hall event on CNN that he and his fellow moderates had not had any talks about one or more of them dropping out. Asked the same question in a post-debate TV interview on Tuesday, Mr. Buttigieg argued that it was he, as the candidate with the second most delegates, whom other moderates should rally behind.

But except for a polling uptick after his strong Iowa finish, Mr. Buttigieg’s support in an average of national polls plateaued around 10 percent. That imperiled him as the race moved to the 14 Super Tuesday states, including California and Texas, where most delegates to the National Convention go only to candidates who win 15 percent in congressional districts and statewide.

As Mr. Sanders, in his second presidential run, built a devoted following of progressives with a call for political revolution, Mr. Buttigieg tried to offer an alternative: an upbeat message of unity and more ideological flexibility, aimed at attracting moderate Democrats, independents and crossover Republicans. But the pitch, which some found contained more platitudes than passion, was no match at a time of rising anger on the left that the political establishment has failed to address health care, income inequality and climate change.

In his quest to earn black support, Mr. Buttigieg spent more time visiting South Carolina than any other candidate, spent more on TV ads in the state than any candidates besides Mr. Steyer, and rolled out a sweeping proposal, called the Douglass Plan, to redress the legacy of racism. None of it made much of a dent with African-American voters who had developed a deep trust in Mr. Biden over decades.

Another factor may have been the sometimes troubled history of Mr. Buttigieg’s relationship with black residents of South Bend, including his demotion of a black police chief and the shooting last summer of a black resident by a white officer. Mr. Buttigieg tried to counter poor impressions by campaigning with African-American leaders from his hometown who vouched for him.

All along, he believed that winning in Iowa would beget winning in later states with more racially diverse voters.

Despite an early exit from the race, Mr. Buttigieg’s candidacy will be remembered for its remarkably high trajectory: the mayor of Indiana’s fourth-largest city outran, out-raised and outpolled senators and governors who dropped by the wayside.

Mr. Buttigieg’s decision, just before Super Tuesday, echoed one he made three years ago during his first foray into national politics. In late February 2017, Mr. Buttigieg dropped out of the contest to become chairman of the Democratic National Committee on the morning of the vote after it became clear he had commitments from fewer than 10 D.N.C. members. After his withdrawal, Mr. Buttigieg received a single vote, from Mayor Nan Whaley of Dayton, Ohio.Reid J. Epstein reported from Selma, Ala., and Trip Gabriel reported from Charlotte, N.C. Alexander Burns contributed reporting from New York.