The announcement caps 15 months of speculation about Mr. Schultz’s intentions. During that span, the Democratic primary field ballooned to as many as 24 candidates and has since shrunk back down to 20. There have been two rounds of nationally televised Democratic debates, with a third debate scheduled for next week.

One of the Democrats in the race, Tom Steyer, is a billionaire like Mr. Schultz. Despite spending millions of dollars of his own money on advertising, he failed to qualify for next week’s debate because of low support in polls.

In June 2018, Mr. Schultz announced he would leave Starbucks after more than three decades leading the company, immediately stoking speculation about a 2020 run. He did little to bat away the possibility at the time, saying he was “deeply concerned” about the country.

Seven months later, in January, Mr. Schultz said that he planned to travel across the country as part of a book tour, and that he had begun the work required to get on the ballot in all 50 states. He said that if did he decide to run, he would do so as an independent, arguing that moderate Republicans and Democrats alike were “looking for a home.”

His move toward a campaign instantly drew criticism from both high-profile Democrats and President Trump, beginning what would become months of public pleading by those who worried that an independent bid would only help Mr. Trump get re-elected by splitting the anti-Trump vote.