The retired three-star admiral can now rightly be called a political giant killer, having slain both longtime U.S. Rep. Curt Weldon and now U.S. Sen. Arlen Specter. Raised as a Catholic, he often invokes the concept of a church schism to warn against the dangers of divisive political discourse, even with Jewish audiences.
From the start of his political career four years ago, the native of Springfield Township in Delaware County has taken pains to build support among local Jews, often invoking his love of midrash and the wisdom of the Talmud.
Now, having ousted Specter, he's got his work cut out for him as he prepares to go up against Republican Pat Toomey in the race for the open Pennsylvania Senate seat in November.
Throughout his two terms in Congress, Sestak has turned up again and again at communal events, from the dedication of a new building for the Jack M. Barrack Hebrew Academy to the opening of the Israeli Consulate's new office in Center City.
In addressing Jewish groups, he rarely fails to mention his admiration for the Jewish faith -- using that term rather than the Jewish people -- and the wisdom of the Torah and Talmud.
"He's a bright man, he's well-read, and that he should know from midrash I think should tell the Jewish community a lot about who he is and how he works," Rabbi Peter Hyman, the former religious leader of Temple Sholom in Broomall, said in an interview last year. Hyman became friendly with Sestak during the lawmaker's first-term.
Sestak has also retold the saga of his daughter's brain tumor, explaining that the reason he got into politics was to ensure that all Americans had access to the same health care that saved his little girl.
When speaking to Jewish groups, he recalls his multiple visits to Israel while in the Navy, along with his commitment to the U.S.-Israel relationship.
But even before his first election to Congress, Sestak has faced questions about his pro-Israel bona fides -- questions that dogged him as he tried to woo Jewish votes in the Senate primary race.
When he first challenged Weldon -- a 10-term lawmaker known for a hawkish approach to foreign policy and one of the House's most ardent Israel supporters -- the incumbent pressed his challenger on the decision to hold a fundraiser with Rev. Robert Edgar, a former House member known as a staunch critic of Israel. Sestak's spokesman at the time called it a nonstory. Indeed, Sestak won handily.
During that first campaign, Sestak outlined his approach to foreign policy -- one that relied heavily on dialogue and diplomacy. He also expressed his concern not only for the security of Israel, but the welfare of Palestinian citizens.
At a 2006 campaign stop at Congregation Beth El-Ner Tamid in Broomall, Sestak said that America needed to bypass the Hamas-led government in order to provide humanitarian assistance in Gaza.
"They are human beings there; their government is wrong," Sestak said at the time.
The issue arose again when he signed on to a congressional letter earlier this year calling on Israel to ease its blockade of Gaza. While the letter cited Israel's security concerns, many critics said that the language was biased against Israel.
But so far, the biggest controversy came in April 2007, just months after he was sworn into office. Sestak accepted an invitation to speak at an event sponsored by a controversial Muslim group -- the Pennsylvania Chapter of the Council on American-Islamic Relations. The appearance was arranged by a Sestak staffer who had formerly worked for CAIR.
Critics accused the organization of being sympathetic to Islamic extremism. Federal prosecutors had labeled it an "unindicted co-conspirator" in a case against the Holy Foundation, whose leaders were charged but never convicted of funneling money to Hamas.
A number of politically influential Jews charged that Sestak was naive or deliberately ignoring facts by agreeing to speak, and pressed him to cancel.
A synagogue in his district, Suburban Jewish Community Center-B'nai Aaron in Havertown, nearly canceled a scheduled Sestak appearance after the controversy erupted. Instead, members took him to task in person for his decision.
Standing on the synagogue's bimah, Sestak defended his right to address constituents, and asserted that he would defend the U.S.-Israel relationship. He also said that more American Muslims need to condemn terrorism.
Irl Barg, a Chester County Sestak supporter involved in a host of Jewish causes, including the American Israel Public Affairs Committee, said that "CAIR was a rookie mistake" on Sestak's part, but added that it took courage to raise Israel and the terrorism issue.
In the end, the episode may have illustrated how Sestak makes a decision and sticks by it, regardless of the consequences. Two years later, when the entire Democratic establishment from the president on down backed Specter and urged Sestak to wait his turn, he rebelled.
Throughout the campaign, Sestak sought to contrast himself with Specter, arguing that the incumbent changed positions and parties to suit his needs.
Sestak was endorsed by J Street in his 2008 re-election bid, but the controversial political upstart sat out the primary.
Now, the question remains: Will Democratic Jewish supporters of Specter who had railed against Sestak back him in a race against the conservative Toomey?
Barg predicts that the candidate "won't be nearly as polarizing to the voting citizens at large" as he was in Jewish circles: "Sestak has good ideas. He's very principled, and he's very pro-Israel."