Out of the running: Sestak announced he was dropping out of the race in a Twitter post on Dec. 1, after failing to gain traction in national polls or qualify for a single Democratic debate.
"I know there is a tear in that fabric right now; but it can be repaired by someone who can lead, and therefore unite, all Americans," Sestak wrote. "Without the privilege of national press, it is unfair to ask others to husband their resolve and to sacrifice resources any longer."
Name: Joe Sestak
Date of Birth: Dec. 12, 1951
Hometown: Springfield, Pennsylvania
Family: Husband to wife Susan and father to daughter Alex
Education: He graduated from the U.S. Naval Academy, earned Ph.D. from Harvard University in political economy and government and a master's degree in public administration
What he does now:
After serving in Congress, Sestak dedicated himself to public service through teaching leadership courses at the U.S. Army War College, Penn State University Schools of Law and International Affairs and other educational institutions throughout Pennsylvania. In an effort to promote the need for science, technology, engineering and mathematics (STEM) education, he brought students together from 160 countries to compete in the first robot Olympics in Washington, according to his campaign website.
What he used to do:
Sestak served in the Navy for 31 years during Vietnam, the Cold War, Iraq and Afghanistan. The retired, three-star admiral was later appointed director of defense policy under the Clinton Administration and assisted in overseeing naval warfare operations. In 2007, he became the highest ranking military officer ever elected to Congress to represent Pennsylvania's 7th Congressional District. Sestak was a member of the Armed Services Committee as well as the Education and Labor Committees, according to his campaign website. He then ran two unsuccessful Senate campaigns.
Key life/career moments:
On Sept. 11, 2001, Sestak was working at the Pentagon when he had stepped outside just before the attack. He subsequently established the Navy's strategic anti-terrorism unit.
During his second Senate run, Sestak campaigned in Pennsylvania by walking 422 miles across the state to hold town halls and speak directly with voters.
Where he stands on some of the issues:
Sestak voted for the Affordable Care Act as well as for Patient Protection while he served in the House. His health care plan aims to expand coverage for even more Americans who are not currently benefiting from the ACA. He aims to lower overall health care costs and increase accessibility for mental health and addition treatment.
His experience on the Armed Services Committee has helped to frame his policy on immigration. His plan targets the threats of the drug trade, gun smuggling and human trafficking while creating a path to citizenship for undocumented immigrants living in the United States.
One of his priorities on environmental policy will be to re-join the Paris Climate Agreement and again establish a relationship with the international community to combat climate change. He said climate change is, "the greatest destructive threat to mankind," in a video announcing his candidacy.
Sestak was not included in the second quarter fundraising figures, as he entered the presidential race at the end of the quarter.
So far he has not qualified for the third debate having come up short on the donor and polling thresholds.
What you might not know about him:
His daughter, Alex, has beaten brain cancer twice, with her first diagnosis coming when she was just 4 years old.
When Sestak was elected to Congress, he became the second Democrat to represent his district since the Civil War.
ABC News' Libby Cathey contributed to this report.