Mr. Sestak, 67 , who served two terms in the House before losing a bid for the Senate in 2010, entered the presidential race in June, far later than most other candidates. He said he had delayed his entry while his daughter received treatment for brain cancer.

But without national name recognition or billions of dollars in personal wealth, Mr. Sestak was unable to make even small inroads in an impossibly saturated field: 24 candidates when he entered.

In October, he walked across New Hampshire — 105 miles in seven days, from Chesterfield to Portsmouth — to try to draw attention to his campaign. In earnest, almost plaintive email missives to supporters, he ticked off the miles, described the scenery and asked for donations.

“The rain is pouring, and the wind is blowing, but I’m still walking across New Hampshire,” he wrote on Oct. 16 under a subject line that began, “Walking in your shoes — in a nor’easter.”