Donald J. Trump's presidential campaign signaled strongly on Thursday that he would name Gov. Mike Pence of Indiana as his running mate, but abruptly postponed a long-planned unveiling of the Republican ticket after an attack that left dozens dead in France.
Mr. Trump said on Twitter that he was delaying his announcement after the "horrible attack" in Nice. He did not specify when the event would go forward.
Before the attack in southern France, Mr. Pence, a mild-mannered Midwesterner popular with conservatives and evangelical Christians, appeared to be all but locked in as the Republican nominee for vice president — the last man standing after a madcap selection process unlike any in recent presidential politics.
But a certain resolution to the process remained elusive after several days of unusually frenzied and public deliberations by Mr. Trump and his family, as well as extraordinarily overt campaigning for the job by several potential running mates. After huddling with Mr. Pence in Indiana, flying multiple other candidates to Indianapolis for last-minute interviews, hinting to party leaders that his decision had been made and then frantically denying it to the news media, Mr. Trump delayed his decision entirely.
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In a television interview, Mr. Trump insisted that he had not settled on a running mate yet. "I haven't made my final, final decision," he told Fox News, speaking by telephone.
Against a backdrop of images from Nice, Mr. Trump repeatedly described himself as the "law-and-order candidate" in the presidential race.
Mr. Trump's advisers told national Republican officials that they were preparing to make an announcement with Mr. Pence, and people close to Mr. Pence notified his political allies that they expected him to be chosen, according to numerous people with knowledge of the conversations, who were not authorized to discuss them publicly.
On Thursday afternoon, television stations in Indiana and New York reported that Mr. Pence had flown from Indiana to Teterboro Airport, arriving in New Jersey late Thursday afternoon for the planned Friday morning event in Midtown Manhattan.
A former congressman and radio host, Mr. Pence emerged over the last week as the strong favorite of Mr. Trump's political advisers and senior officials in the Republican Party. He addressed a rally in Indiana alongside Mr. Trump on Tuesday night and met privately with him several times.
But Mr. Trump himself has sent conflicting signals in recent days, as he has subjected his potential running mates to a final round of screening. As of Thursday afternoon, he had not yet formally invited Mr. Pence to join his ticket, nor had he notified two other contenders, former House Speaker Newt Gingrich and Gov. Chris Christie of New Jersey, of his decision, according to people with direct knowledge of the process.
Mr. Trump has appeared to vacillate over his choice. Long accustomed to making important strategic decisions by sheer improvisation, he is now faced with the most permanent, and perhaps most important, decision of the campaign.
The delay has the potential to complicate a partnership with Mr. Pence, who must file papers in Indiana by noon on Friday withdrawing from his re-election campaign in order for Republicans to field a new candidate for the race.
On Wednesday, Mr. Trump and his children threw together a hasty series of conversations with other finalists in the vice-presidential search, including Mr. Christie, Senator Jeff Sessions of Alabama and Mr. Gingrich.
Paul Manafort, the chairman of the Trump campaign, said at midday Thursday that the campaign had not notified anyone of a final decision.
"We have not been reaching out to Washington to tell them to prepare for any particular candidate," Mr. Manafort said by phone.
A low-key man largely defined in public life by his Christian faith, Mr. Pence, 57, is seen as a cautious choice — a political partner who is unlikely to embarrass Mr. Trump, and who may help him shore up support among conservative voters still wary of his candidacy.
Mr. Pence's staunch conservative views on certain social issues, like gay rights and abortion, may inject a new set of concerns into the general election debate that have been largely overlooked with Mr. Trump at the top of the Republican ticket.
Mr. Gingrich, in a video broadcast on Facebook, said Mr. Pence would bring "Midwestern appeal" to the Trump ticket, and would pair Mr. Trump with a "relatively stable, more normal person."
Mr. Gingrich added that he had told Mr. Trump that a Trump-Gingrich combination would be a bolder pairing of "two pirates" on the same ticket.
Selecting Mr. Pence, by contrast, might ease relations between Mr. Trump and Republicans in Washington, where party leaders have eyed his every move with grave apprehension.
Republicans on Capitol Hill spoke approvingly of Mr. Pence on Thursday: He is seen among his former colleagues there as a conventional politician with standard-issue conservative beliefs, including on some subjects where his policy instincts plainly conflict with Mr. Trump's.
Mr. Pence has endorsed free-trade agreements, including the Trans-Pacific Partnership, an Asian trade deal that Mr. Trump has described as a "rape" of the American economy. As a House member, Mr. Pence also voted for the Iraq War, which Trump has condemned, and last winter he denounced Mr. Trump's call to ban all Muslim immigration into the United States.
If those views place Mr. Pence at odds with Mr. Trump, they are in line with the outlook of Republican leaders in Congress.
"It's no secret I'm a big fan of Mike Pence's," said Paul D. Ryan, the speaker of the House from Wisconsin. "We're very good friends. I have very high regard for him. I hope that he picks a good movement conservative. Clearly, Mike is one of those."
The Senate majority leader, Mitch McConnell, of Kentucky, said picking Mr. Pence would be a "good move by Donald Trump." Mr. McConnell, who has sharply rebuked Mr. Trump in recent weeks for his indiscipline on the campaign trail, said he would "look forward to enthusiastically supporting the ticket."
To Democrats, Mr. Pence cuts an unimposing profile, and party officials said Thursday they considered him unlikely to transform Mr. Trump's deeply unpopular public image. On the contrary, his arch conservative social views could help motivate liberal voters and young people to turn out for Hillary Clinton in the fall.
At the same time, Democrats have looked with some dismay at Mr. Trump's tenacity in the Midwest, and at his unusual popularity with white men. Mr. Pence could reinforce Mr. Trump's strengths in both areas.
The Senate minority leader Harry Reid, Democrat of Nevada, indicated that the focus of the fall campaign would be Mr. Trump himself.
"It's not going to help him that much, no matter who he picks," Mr. Reid said.
For Mr. Trump, selecting Mr. Pence would be a sharp departure from habit, and the surest sign yet that he intends to submit to at least some standard political pressures in the general election.
Mr. Pence's public audition for the No. 2 spot, when he appeared with Mr. Trump on Tuesday at a rally in Westfield, Ind., went well. Standing ramrod straight, Mr. Pence offered five and a half minutes of high-energy remarks, frequently turning his fire to Hillary Clinton in an apparent attempt to demonstrate that he could be an attack dog against her.
But in the past, Mr. Trump has leaned heavily on a tiny circle of trusted friends and advisers, and has crafted his major political decisions to shock and titillate the news media and Republican primary voters.
Mr. Pence is a laid-back personality who does not have the same set of showman's instincts as Mr. Trump or other vice-presidential contenders like Mr. Gingrich and Mr. Christie.
And the Indiana governor has only the scantiest of personal relationships with the man whose political future has rapidly melded with his own.
— David M. Herszenhorn, Jonathan Martin, Alan Rappeport and Katie Shepherd contributed reporting.