Donald J. Trump’s campaign was teetering early last month, with an increasingly isolated candidate and a downcast staff that seemed to lurch from crisis to crisis. Having fired his campaign chairman and retooled his message, Mr. Trump was still far behind Hillary Clinton in the polls, and Republicans were running away from him.

Under those desperate conditions, Mr. Trump’s closest allies last month pressed him to approve a daring plan: Go to Mexico and meet with President Enrique Peña Nieto, presenting himself to the world as a statesman and earning a new look from millions of American voters.

But the political gymnastics involved in Mr. Trump’s gambit will likely be difficult to sustain: His approach involves avoiding discussion of his former campaign pledges without renouncing them, and making ostentatious gestures of conciliation toward Hispanic voters and Mexicans without withdrawing — and at times actually repeating — remarks that have offended them in the past.

In the space of a few hours on Wednesday, Mr. Trump veered from avoiding a clash with Mr. Peña Nieto over his proposal for a border wall to goading an Arizona crowd into chants about constructing the barrier.

If winning over people who view him as a racially divisive or reckless candidate would seem to require a dizzying political reinvention, it is far from certain that Mr. Trump is prepared to transform himself so thoroughly.
A supporter at Mr. Trump’s speech in Phoenix on Wednesday.
Travis Dove for The New York Times

He appeared solicitous, even pleading, in his visit to Mexico City, shirking confrontation with Mr. Peña Nieto and reading slowly from a cautious, tightly phrased statement that described his admiration for Americans of Mexican descent.

“Spectacular, spectacular, hard-working people,” Mr. Trump said of Mexican-Americans. “I have such great respect for them and their strong values of family, faith and community.”

In Arizona, Mr. Trump made his most brazen attempt yet to back away from his pledge to deport all 11 million undocumented immigrants, denouncing illegal immigration in vehement terms, while at the same time revising his policy agenda. Where he has, in the past, suggested creating a special force to achieve that goal, Mr. Trump said on Wednesday that a new “deportation task force” would focus on rounding up only the “most dangerous criminal illegal immigrants.”

Underscoring the new approach, Mr. Trump was introduced in Phoenix by a pair of loyal supporters, Rudolph W. Giuliani, the former mayor of New York, and Senator Jeff Sessions of Alabama, who took turns wearing a baseball cap bearing the slogan: “Make Mexico Great Again Also.”

Mr. Trump has already sought several times to reboot his campaign and reintroduce himself to general-election voters, many of whom hold him in low regard. He has periodically reworked his message to conform more closely to conventions of political discourse, briefly earning praise from Republicans and critics in the media, only to lapse soon after into old, unrestrained habits.

On Wednesday night, as the crowd in Phoenix grew more energized, he could not resist returning to his fiery form, even as he outlined his new approach to immigration control.

He repeated at high volume his harsh denunciations of illegal immigration as a threat to public safety. Mrs. Clinton’s plan, he said, was “open borders and let everybody come in and destroy our country.”

At one point, referring to Mrs. Clinton, he told the crowd that perhaps he should “deport her.”

And Mr. Trump, as is his pattern, created confusion for even his closest supporters as he appeared to embrace opposite sides of important issues as the day unfolded.

He told reporters in Mexico that he and Mr. Peña Nieto had not discussed forcing that country to pay for a border wall, suggesting the delicate question would be explored in the future by the two leaders. But hours later, Mr. Trump thundered in Phoenix that his mind was made up: Mexico would foot the bill.

Mr. Trump’s course adjustment emerged in an atmosphere of growing urgency and alarm within his campaign. Over the last week, close associates have told both Mr. Trump and members of his family that he is in real danger of losing the race, according to a half-dozen people close to the Trump campaign and briefed on its activities, who spoke on condition of anonymity to avoid angering the nominee.

Gov. Chris Christie of New Jersey, who lobbied in favor of the trip to Mexico, has told Mr. Trump that he must make immediate changes to regain his footing, associates said. And on Monday, Mr. Trump’s son Eric met with senior officials at the Republican National Committee in Washington and heard a grim prognosis of his father’s campaign, according to people briefed on the meeting.

Without a major shake-up of the electoral map, strategists indicated to the younger Mr. Trump, his father’s already narrow path to the 270 electoral votes he needs to win could vanish. Going through the swing states one by one, party officials showed Eric Trump that his father was drastically underperforming other Republicans in the polls.

Ahead of his travels on Wednesday, Trump aides told his political allies that the campaign hoped to provide a new template for talking about immigration for the remainder of the race: One that would continue to stress law-and-order themes and nationalistic passions, while abandoning his specific commitment to quickly deport 11 million migrants.

The idea of a trip to Mexico had been under consideration for weeks: Melania Trump, Mr. Trump’s wife, endorsed the move earlier in August and Jared Kushner, Mr. Trump’s son-in-law, had raced to open talks with the Mexican foreign ministry. Yet Mr. Trump himself had been hesitant, fearing a fiasco if Mr. Peña Nieto attempted to embarrass him.

Mr. Giuliani, who has become one of Mr. Trump’s chief counselors, and Mr. Christie urged him to make the trip and ultimately helped craft the statement he delivered in Mexico City.

There were holdouts in Mr. Trump’s inner circle: Roger Ailes, the former chairman of Fox News who is now advising Mr. Trump, saw the idea as needlessly risky.

By Monday morning, however, Mr. Trump decided that his political circumstances demanded bold action.

Dan Senor, a former adviser to Mitt Romney’s 2012 campaign who has been critical of Mr. Trump, said the trip to Mexico likely helped elevate Mr. Trump in the eyes of some voters. “The expectation was so low that simply showing up on the stage next to a head of a state was a win for him,” Mr. Senor said.

Still, with his decision to seek reinvention in a whirlwind trip, Mr. Trump signaled on Wednesday that any effort to run a cautious or conventional general-election campaign is clearly behind him. And with less than 10 weeks to go in the race and a darkening political landscape confronting him, Mr. Trump’s brand of disruptive spectacle may increasingly come to define his candidacy.

Trailing Hillary Clinton, Donald Trump Turns to Political Gymnastics