”I have no regrets,” he added later. “I’m excited about what we’ve done.”

Mr. Swalwell had been scheduled to visit New Hampshire and attend several events there on July 3 and 4, but a campaign spokeswoman told The Times last week that he had canceled his plans and instead spent the holiday with family.

Ending his presidential campaign early will allow Mr. Swalwell to shift his attention to seeking a fifth term in the House representing Northern California’s East Bay. He had previously hinted that he would assess his standing in the presidential primary in December — California’s registration deadline to run for Congress — and make a decision about a House run at that time. But during his news conference Monday, he confirmed that he would seek re-election.

[The On Politics newsletter looks at which 2020 candidates may be eyeing other races.]

Mr. Steyer, on the other hand, has no other clear electoral prospects. Though he has repeatedly toyed with running for high office, including the governorship of California and a Senate seat there, this moment appears to be all about entering the Democratic primary field.

In recent weeks he instructed advisers to conduct polling and other research to gauge his prospects, people briefed on his preparations said. He has designated a longtime aide, Heather Hargreaves, as his campaign manager-in-waiting, and recorded a television ad that could air in the early voting states in the event he becomes a candidate. That launch, associates said, could happen as soon as this week.