Joe Sestak, the former admiral and congressman, walks through Pittsburgh on March 26, toward the end of his month-long walk across Pennsylvania. (Jason Cohn for The Washington Post)

To explain why he was walking alone for 25 days and 422 miles across Pennsylvania, U.S. Senate hopeful Joe Sestak recalled a bit of advice he got six years ago.

It was the day that Sen. Arlen Specter became a Democrat, a move that thrilled the party but complicated Sestak's plans to run for the seat. That afternoon, Sestak had lunch with his old boss, Bill Clinton. The former president, Sestak says, jabbed a finger in his gut — and told him to follow it.

On a gray, chilly weekday afternoon six years later, his gut was leading him up a steep hill in a working-class Steel City neighborhood, 12 or so miles and 12 or so hours into Day 23 of his journey. Near the top, he spotted a potential voter, a man in baggy gray sweatpants and a black zippered hoodie stepping out of a black SUV, cigarette and Monster energy drink in hand.

Sestak beelined for him. He showed the man a sheet of paper — a printed Google map. Was he headed in the right direction?

Sestak vaguely explained that he was a former congressman who "beat someone but then lost to someone else" and was now walking — yes, walking — the Keystone State. He got the man to pose with him for a selfie, handed him a business card with his personal e-mail address ("just say you're the guy who gave me directions to Steuben Street") — and then he was off again, a solitary figure hustling forth on a vision quest of his own.

The journey could be a metaphor for his politics — a go-it-alone strategy that challenges norms and only occasionally pays off. Take that follow-your-gut moment of his from six years ago: Sestak forged ahead and challenged Specter in Pennsylvania's 2010 Democratic primary for Senate — but in the process alienated Democratic leaders who coalesced behind Specter. (Clinton himself was deputized to call Sestak and ask him to drop out.) Sestak won the primary, only to lose narrowly in the general election to Republican Patrick J. Toomey.

Now he is embarking on another maverick campaign to challenge Toomey in 2016. But while he is the unofficial front-runner on the Democratic side, he is carrying the baggage of 2010. National and state Democrats remain uneasy about a guy who they feel bucked their authority the last time and whose initial campaign strategy is to walk alone for a month — so much so that they are practically begging other Democrats to run against him.

On a windswept bridge over the Monongahela River, Sestak paused for a moment to reflect on his strained relations with his party. He insists he is untroubled by it. When he faced tea party opposition in 2010, he still made a point of getting to town halls early to shake protesters' hands, he said.

"So how could I not go down and shake the hands of the DSCC [Democratic Senatorial Campaign Committee] and say, 'Can we work together?' "

"But if not," he added, "I understand if they have a better choice."

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His day started at 6 a.m. with a drop-off at an empty parking lot across from a Dollar General. Exactly where he was picked up at the end of the previous day. In darkness, with a backpack slung over his right shoulder, Sestak began to walk.

Over the next 19 hours, he would walk 18 miles, with breaks for a campaign event, four media interviews and a visit with college students.

Sestak, 63, began walking in early March at Independence Hall in Philadelphia — near Delaware County, where he grew up and now lives with his wife and teenage daughter. He had averaged about 20 miles a day across Amish farmland, up icy mountains and along the shoulders of narrow thoroughfares on his journey to the Ohio border.

On this late March Thursday in Pittsburgh, he crossed into an upper-middle-class neighborhood dotted with churches and old mansions, the sidewalks vacant except for a few early morning joggers. He turned onto Fifth Avenue and went through the heart of the University of Pittsburgh campus. Students hustled past, not seeming to notice the slender, gray-haired man in an olive Navy flight jacket. They would have had no idea he is a retired three-star admiral and former member of Congress with his sights on a return to Capitol Hill, and he did not stop to tell them.

This journey, it sometimes seemed, was as much about solitude and self-reflection as retail politics or media outreach. He rarely slowed to