Senator Chuck Schumer, the Democratic minority leader, and Graham Platner, the party’s now presumed candidate in Maine’s crucial Senate race, talked by telephone on Thursday after Gov. Janet Mills, Mr. Schumer’s favored candidate, abruptly suspended her campaign.

A person familiar with the call would provide few details when asked about any contact between the two men, but acknowledged they had spoken and said they agreed that Democrats needed to win Maine and take back the Senate majority from Republicans. Another person familiar with the conversation said it was cordial and stressed the importance of unseating Senator Susan Collins, the five-term Republican incumbent.

It was the first exchange between them. Mr. Schumer, the New York Democrat long deeply involved in his party’s Senate campaign strategy, had heavily recruited Governor Mills and saw her as the party’s best opportunity to defeat Ms. Collins, the chairwoman of the Appropriations Committee who has repeatedly fended off Democratic challengers.

But Mr. Platner, a populist oysterman and political novice, proved formidable despite Governor Mills’s political pedigree. Mr. Schumer now finds himself trying to establish a relationship with the party’s presumed candidate in what is likely a must-win race if Democrats are to have any chance of taking the Senate.

The decision by Governor Mills was a significant setback for Mr. Schumer that underscored discontent about him among some in the party and had some Democrats questioning whether he had lost a step in his political calculations. But he struck an optimistic tone as he left the Capitol on Thursday night.

“We are going to beat Susan Collins and we are going to win back the Senate,” he told reporters.

Democrats believe that voter discontent in Maine with the direction of the Trump administration gives them a strong chance to defeat Ms. Collins, who has proved politically resilient at home.

Annie Karni contributed reporting.