On Wednesday, Donald J. Trump faced unusual public attacks from across the Republican Party in the aftermath of a string of midterm losses by candidates he handpicked and supported, a sign of weakness as he prepares to launch a third presidential campaign as soon as next week.

The rush to openly blame Mr. Trump was as immediate as it was surprising as the sheer number of missed Republican opportunities became clear.

Conservative allies slammed Mr. Trump on social media and cable news, questioning whether he should remain the party’s leader and pointing to his toxic political brand as the common thread woven through three consecutive election cycles.

Mr. Trump was widely blamed for the Republicans’ poor showing in Tuesday’s elections, as a number of candidates he backed in competitive races were defeated, including nominees for governor and Senate in Pennsylvania, as well as governors in Michigan, New York, and Wisconsin.

Former Long Island Republican Representative Peter King, who has long supported Mr. Trump, said, “I strongly believe he should no longer be the face of the Republican Party,” adding that the party “cannot become a personality cult.”

The outpouring of criticism on Fox News and social media throughout the day revealed Mr. Trump to be at his most politically vulnerable point since the aftermath of the Jan. 6, 2021, attack on the Capitol.

Nonetheless, Mr. Trump has built a deep well of loyalty among Republican voters, and party officials warned that it was too early to tell whether he would suffer any long-term political damage beyond a flurry of bad headlines, or whether a rival would emerge to challenge him. Mr. Trump’s career has been built on surviving political controversy, and Trump aides insisted that any suggestion of weakness was a media fabrication.

Senator-elect J.D. Vance, a Republican from Ohio and an early supporter of Mr. Trump, stated that if he runs, he believes he will be the nominee. “The media writes Donald Trump’s political obituary every year.” “And every year, we are reminded that Trump is still the most popular figure in the Republican Party,” he said. And Representative Jim Banks of Indiana said he supported Mr. Trump, who “transformed our party.”

After The New York Times sought comment from a Trump aide, Ms. Stefanik, Mr. Vance, and Mr. Banks all issued statements.

Mr. Trump put the best face forward publicly, pointing to dozens of victories for his endorsed candidates in less competitive races.

In an interview with Fox News on Wednesday, he cited Mr. Vance, who won convincingly, and Herschel Walker, the former football star, who will face Senator Raphael Warnock in the Georgia runoff.

Mr. Trump, however, was privately blaming others, including Sean Hannity and casino mogul Steve Wynn, for his endorsement of Mehmet Oz, the defeated Pennsylvania Senate candidate. According to several people familiar with the discussions, he included his wife, Melania, among those he claimed had given him bad advice.

A few Republican operatives who had previously expressed interest in working with another Trump presidential campaign have changed their minds. That could pose a problem for Mr. Trump, who has a few trusted advisers but almost no one working on key aspects of a campaign-in-waiting.

Kayleigh McEnany, a former Trump White House press secretary and one of his staunch supporters, said on Fox News on Wednesday that her former boss should wait until after the Georgia Senate runoff election to make an announcement.

Mr. Trump, on the other hand, has been teasing rally crowds for weeks with hints of another presidential bid — one meant to capitalize on the momentum gained by what he repeatedly predicted would be a towering Republican victory in the elections on Tuesday. That would allow him to claim credit for endorsing the winners, holding dozens of rallies to highlight them, and spending millions of dollars from his campaign treasury on advertisements in their support.

Instead, the party fared far worse than expected, though it remains within striking distance of controlling one or both houses of Congress.