Citing Economy, Trump Says That ‘You Have No Choice but to Vote for Me’

Before his ouster, Mr. Lewandowski helped Mr. Trump notch his first primary victory, in New Hampshire, which helped vault him to the Republican presidential nomination. Mr. Lewandowski would hope to run on the same outsider energy that Mr. Trump channeled that year.

At the same time, Mr. Lewandowski has significant vulnerabilities. On Thursday, hours before the Trump rally, the House Judiciary Committee subpoenaed him in relation to the report by the special counsel, Robert S. Mueller III. His opponents would be almost certain to raise questions about his business activities since Mr. Trump took office.

Gov. Chris Sununu of New Hampshire, a Republican who is up for re-election next year, has told the White House he has concerns about the effect that Mr. Lewandowski’s candidacy could have on others on the ballot, according to two people briefed on the discussions.

The Republican primary race already has several declared candidates, including the retired Brig. Gen. Donald C. Bolduc and Bill O’Brien, the former state House speaker.

The rally in Manchester, a southeastern city of 110,000, drew people from several nearby states. At the front of the line to get into the rally, a cluster of people who had been there from the night before agreed that the president’s stance on immigration was a key reason they would be voting for him again in 2020.

“Illegal immigrants impact every community,” said Nena Alexandra, 20, from Easthampton, Mass. “That goes for every single family in the United States.”

But even among the die-hards at the front of the line, there was some detectable unease as the stock market reeled. Kevin Steele, 40, a self-described Democrat for Trump, said he hoped that Mr. Trump’s trade war would be over soon.

“Listen,” Mr. Steele, who is unemployed and lives in Scranton, Pa., said, “I’m hoping to God he closes with China. Just get the deal done. All of my money’s in stocks.”