Vice President Biden on Wednesday announced he is not running for president in 2016, forgoing a primary battle against Democratic front-runner Hillary Clinton and likely signaling an end to a political career that has spanned four decades.

Ending months of speculation about his plans, Biden said he could not see a path forward for his candidacy. The window of opportunity, he declared, “has closed.”

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“Unfortunately, I believe we’re out of time — the time necessary to mount a winning campaign for the nomination.”

The 72-year-old vice president made the announcement from the White House Rose Garden with his wife, Jill, and President Obama at his side.

Biden did not go into detail about the thinking behind his decision, though many suspected that the resurgence of Clinton — fueled by Republican gaffes, her rising poll numbers and a spirited debate performance — had dampened the Democratic appetite for an alternative.

Citing the “inspiration” of his late son Beau Biden, the vice president used his moment in the spotlight to deliver the speech he might have given as a candidate, with an emphasis on making Washington “part of the solution.”

“I believe we have to end the divisive partisan politics that is ripping this country apart. And I think we can. It’s mean-spirited. It’s petty. And it’s gone on for much too long.”

Biden did not mention Clinton once in the 14-minute remarks, though he delivered a clear rebuke of her quip that she’s proud to have made enemies in the GOP.

“I don’t believe, like some do, that it’s naive to talk to Republicans. I don’t think we should look at Republicans as our enemies. They are our opposition. They’re not our enemies. And for the sake of the country, we have to work together.”

Biden’s decision to opt out of the race, which an aide said he made on Tuesday night, solidifies Clinton’s standing as the likely Democratic nominee.

The vice president was perhaps the only Democrat who could have mounted a serious challenge to Clinton despite the threat she faces from liberal Sen. Bernie Sanders (I-Vt.) in the early-voting states of Iowa and New Hampshire.

A few weeks ago, Biden appeared to have a real opening to enter the contest, with the Clinton campaign struggling to contain the controversy over her use of a private email server while secretary of State.

The possibility of a Biden run stirred alarm in the Clinton camp. The candidate appeared energized by the threat, and made aggressive moves to the left while ramping up her attacks on the Republican field.

The turning point appeared to be last Tuesday’s Democratic debate, when Clinton commanded the stage and erased many doubts about her readiness for a general election battle.

Polls released after the debate indicated Clinton improved her standing with voters, expanding her lead over Biden in a hypothetical match-up. Top Democrats began to question whether Biden had missed his moment.

The vice president appeared to undermine his chances further on Tuesday, when he denied opposing the raid that killed Osama bin Laden. The comments contradicted his previous statements that he had advised against the risky mission — one of Obama’s most consequential decisions in office.

Despite bowing out of the race, Biden made clear Wednesday that he’s not ready to throw his support behind Clinton. The vice president said it’s crucial for Democrats to defend Obama’s legacy — and implicitly his legacy as well.

“Democrats should not only defend this record and protect this record, they should run on this record.”

Clinton, who reportedly spoke to Biden after his Rose Garden announcement, said he would continue to be a force within the Democratic Party.

“I am confident that history isn’t finished with Joe Biden Joe BidenOmar fires back at Trump over rally remarks: 'This is my country' Trump mocks Biden appearance, mask use ahead of first debate Trump attacks Omar for criticizing US: 'How did you do where you came from?' MORE,” she said in a statement. “As he said today, there is more work to do. And if I know Joe, he will always be on the front-lines, always fighting for all of us.”

Much of the Democratic Party appeared relieved by Biden’s decision, in no small part because of the significant doubts about his chances.

He would have begun the race with zero dollars in the bank, while Clinton has raised a whopping $77 million since launching her campaign in the spring.

He would have had difficulty stealing the populist mantle from Sanders and could have struggled to create a contrast with Clinton, with whom he served in the Obama administration for four years.

Biden’s entry also would have put Obama in an agonizing position, caught in the middle of a contest between two of his political allies and friends.

Still, some Democrats clamored for Biden to jump into the race, saying his off-the-cuff style would have offered an appealing alternative to the scripted Clinton.

Biden vowed Wednesday that he would not be “silent” during his final months in the White House and said one of his priorities would be seeking a “moonshot” to cure cancer, which has become “personal” after the disease took his son.

“I intend to speak out clearly and forcefully, to influence as much as I can where we stand as a party and where we need to go as a nation.”

Reflecting on his life, which has included positions of power in the Senate and two failed runs for the presidency, Biden spoke of his gratitude for the “privilege” of being in public service.

The Biden family, he said twice for emphasis, “found purpose in public life.”

—This story was updated at 8:36 p.m.