US presidential candidate who conceded the election to his opponent when he had actually won?

1824.

“Andrew Jackson won a slight plurality in the popular vote, leading John Quincy Adams by 38,149 votes. Four candidates received Electoral votes, though none received enough to constitute a majority:

Andrew Jackson received 99 Electoral votes.
John Quincy Adams received 84 Electoral votes.
William H. Crawford received 41 Electoral votes.
Henry Clay received 37 Electoral votes.

Since there was no majority within the Electoral College, the decision was deferred to the House of Representatives. The House is only allowed to vote on the top three contenders from the Electoral College so Henry Clay was removed from the election.

Adams, who was Jackson’s most viable competition, sought Clay’s support, knowing it would bring him victory. As the vote neared, Clay worked hard rounding up support for Adams. He won over Western representatives whose states had voted solidly for Jackson and even promised the votes of his home state Kentucky, which had not cast a single popular vote for Adams. After more than a month of bargaining, John Quincy Adams took precisely the 13 states he needed to win, Jackson won seven, and Crawford won four. ”

Source

Don’t know if this is what you’re referring to, but in 1948, when President Harry S Truman (Democrat) ran against Thomas Dewey (Republican), the Chicago Tribune famously predicted “Dewey Defeats Truman” in a front-page headline that obviously got it wrong. Story at Wikipedia. I don’t think that Truman actually conceded, however, before the election returns were finalized. See photo of Truman holding up a copy of the newspaper, grinning ear to ear.

@marinelife you forgot to add Al Gore to your list. Bush was appointed as president by the supreme court. I doubt that was a constitutional decision.

George W Bush.

@Ron_C

if you mention Gore/Bush you need to also mention Kennedy/Nixon when Nixon won the pop vote and Kennedy was appointed.

@DrBill really? Was Kennedy appointed by the supreme court or did he just win the electoral college?

@Ron_C

You may want to refresh your memory on what the Supreme Court actually decided. Despite the recounts conducted after the election that all showed Bush the winner, Democrats continue to whine about the outcome.

The court ruled that the inconsistent methods being employed violated the Equal Protection Clause. The rules for determining Voter Intent were applied differently from district to district and even withing districts. The selection of which votes to recount were likewise applied differently. It was a blatant attempt by the Democrats to manipulate election results and couldn’t be condoned. Bush won on the original count, he won on the recount, and he won on all subsequent recounts. Let it go.

@DrBill Kennedy won both the popular vote and the Electoral College vote.

@Jaxk you might want to rethink your position on who actually suppressed the vote. Florida and Ohio had particularly egregious violations in not in law but definitely in spirit in voter suppression. Everything from making sure that there were long lines, early closings, and short supplies in areas likely to vote democrat to the “hanging chad” disaster with Florida punch cards. I think there should have been a run-off. Bush petitioned the court to protect him from the “irreplaceable harm” in continuing the count. It is clear the 43rd president of this country was selected by the Supreme Court.

The story sounds like a reworking of the Truman/Dewey election of 1948:
Truman did not concede, however. From Wikipedia:
“As the returns came in Truman took an early lead which he never lost. However, the leading radio commentators, such as H. V. Kaltenborn of NBC, confidently predicted that once the “late returns” came in Dewey would overcome Truman’s lead and win. At midnight, Truman awoke and turned on the radio in his room; he heard Kaltenborn announce that, while Truman was still ahead in the popular vote, he couldn’t possibly win. Around 4 a.m. Truman awoke again, heard on the radio that his lead was nearly two million votes, and decided to ride back to Kansas City. For the rest of his life, Truman would gleefully mimic Kaltenborn’s voice predicting his defeat throughout that election night. Dewey, meanwhile, realized that he was in trouble when early returns from New York and New England showed him running well behind his expected vote total. He was also troubled when the early returns showed that Henry A. Wallace and Strom Thurmond, the two third-party candidates, were not taking as many votes from Truman as had been predicted. Dewey stayed up throughout the night examining the votes as they came in. By 10:30 the next morning he was convinced that he had lost; he then sent a gracious telegram of concession to Truman.”

I am unaware of any elected U.S. President conceding defeat before election and then withdrawing the concession to take office. The only candidate that I am aware did so, before contesting results, is Al Gore.