WASHINGTON — In her first public remarks since becoming the de facto presumptive Democratic nominee for president less than 24 hours earlier, Vice President Kamala Harris on Monday morning did not mention that fact and only obliquely alluded to how it had come to pass.

“I am firsthand witness that every day our president, Joe Biden, fights for the American people. And we are deeply, deeply grateful for his service to our nation,” she said at the start of a brief speech honoring NCAA championship athletes gathered on a hot and humid late morning on the White House South Lawn.

Biden ordinarily would have hosted such a reception, but he remains at his home in Rehoboth Beach, Delaware, recovering from a new bout of COVID. On Sunday afternoon, he announced that he was dropping his reelection effort and was throwing his support behind Harris to succeed him. Polls had shown that an overwhelming majority of Americans believed that Biden, who is 81, was simply too old to seek a second term.

While Harris did not acknowledge her new stature from behind a lectern bearing the vice presidential seal, the hordes of reporters gathered behind the seated audience — many, many times the number who ordinarily cover her events ― attested to her sudden importance.

As the event was unfolding in the light drizzle, Harris’ office announced that she would later in the afternoon travel to Wilmington, Delaware, Biden’s home and site of his reelection campaign. It will be the first event with her at the top of the ticket, rather than as the running mate.

Harris has traveled the nation on White House business since the start of the Biden administration in January 2021, but in recent months has stepped up her schedule of appearances, with many of them for the campaign. She has been the Biden-Harris campaign’s main messenger on protecting abortion rights, and, with her background as a prosecutor, has recently been attacking Republican nominee Donald Trump for the actions that led to his various criminal prosecutions.

In a post to social media Sunday, Trump hinted at backing out of a September presidential debate that was to be hosted by ABC News — suggesting that he does not want to face Harris one-on-one.

Trump formally accepted the GOP nomination at the party’s convention in Milwaukee last week. His speech focused on attacking Biden, and both he and his allies have spent the last day complaining that it is unfair for Democrats to switch their presidential nominee after Trump has already spent many millions of dollars and an entire year and a half attacking Biden.

The complaints seem to belie top campaign adviser Chris LaCivita’s claim at a Republican National Convention event last week that it made no difference whom the Democrats nominated and that the Trump camp was equally prepared to run against Harris. “We’re prepared for whatever,” he said. “If it happened, the national press will do their best to hand Kamala a reset button, but we won’t allow that.”