WASHINGTON » Vice President Joe Biden will not run for president in 2016, he said today, ending a months-long flirtation with a third White House campaign and setting him on a glide path toward the end of his decades-long political career.

Biden’s decision finalizes the Democratic field of White House candidates and bolsters Hillary Rodham Clinton’s standing as the front-runner by sparing her a challenge from the popular vice president.

In an extraordinary appearance in the White House Rose Garden, Biden said he always knew the window for a viable campaign might close before he could determine whether his family was emotionally prepared for another campaign so soon after losing his son, Beau Biden, in May. Biden said his family was prepared to back him, but that he nonetheless would not be a candidate.

"Unfortunately, I believe we’re out of time," he said, flanked by President Barack Obama and Biden’s wife, Jill.

Encouraged by Democrats seeking an alternative to Clinton, Biden had spent the past several months deeply engaged in discussions with his family and political advisers about entering the primary. Yet as the deliberations dragged on, Democrats began publicly questioning whether it was too late for him to run, a notion that hardened after Clinton’s strong performance in last week’s Democratic debate.

Notably, Biden did not endorse Clinton or any of the other Democratic candidates. Instead, he used the announcement to outline the path he said Democrats should take in the 2016 campaign, including a call for them to run on Obama’s record. In what could have been a campaign speech, Biden deplored the influence of unlimited contributions on politics, called for expanding access to college educations and called on Democrats to recognize that while Republicans may be the opposition, they are "not our enemy."

"While I will not be a candidate, I will not be silent," Biden said.

today’s announcement was a letdown for Biden supporters who had pleaded with him to run, and in increasingly loud tones as his deliberations dragged on through the summer and into the fall.

For months, the 72-year-old Democrat made front pages and appeared on cable news screens as pundits mused about his prospects and Clinton’s perceived vulnerability. A super political action committee, Draft Biden, was formed with the explicit goal of getting him into the race.

At the White House, aides and longtime Biden loyalists had prepared for a potential bid, putting together a campaign-in-waiting should he decide to jump in. Last week one of those aides, former Sen. Ted Kaufman, wrote an email to former Biden staffers laying out the potential rationale for a Biden run and promising a decision soon.

Biden spoke personally to many supporters. As speculation about his plans reached a fever pitch, he kept up an intense schedule of public appearances, seemingly testing his own stamina for an exhausting presidential campaign.

But he also continued to broadcast his reluctance amid doubts that he and his family were emotionally ready in the wake of Beau Biden’s death.

In a September appearance on "The Late Show," Biden told host Stephen Colbert he was still experiencing moments of uncontrollable grief that he deemed unacceptable for a presidential aspirant. "Sometimes it just overwhelms you," he said, foreshadowing his ultimate decision.

Biden would have faced substantial logistical challenges in deciding to mount a campaign this late in the primary process.

Both Clinton and Vermont independent Sen. Bernie Sanders have been in the race since April — giving them a powerful head start in fundraising, volunteers, endorsements and voter outreach. Top Democratic operatives and donors already committed to Clinton would likely have had to defect to Biden in order for him to have viable shot at the nomination.

Having decided against a final presidential campaign, Biden now approaches the end of his long career in politics.

A month after being elected to the Senate in 1972 at age 29, Biden’s wife and baby daughter died when their car collided with a tractor-trailer. Biden considered relinquishing his seat, but instead was sworn in at the hospital where his sons, Beau and Hunter, were recovering.

Over six terms in the Senate, he rose to become chairman of the Senate Judiciary and Foreign Relations committees, developing broad expertise in global affairs and a reputation for a plainspoken, unpredictable approach to politics.

Biden twice ran for president. His most recent attempt in 2008 ended after he garnered less than 1 percent in the Iowa caucuses. His first run in 1987 ended even more quickly, following allegations he plagiarized in some speeches from a British politician.

He has not yet detailed his post-White House plans, but has told friends he has no plans to retire in a traditional sense. Although unlikely to again seek elected office, friends and aides say Biden has previously discussed starting a foundation, launching an institute at the University of Delaware or taking on a role as a special envoy and elder statesman if called upon by future presidents.