Vice President Joe Biden will not seek the presidency, a decision that will solidify Hillary Clinton's position as the heavy favorite for the Democratic nomination.

Biden announced his decision not to run from the White House, after several weeks in which groups supporting his run pour millions of dollars into the effort to lure him into the race.

Biden told reporters that he believed the "window has closed" on what would have been his third try for the Democratic nomination. Biden's announcement comes just three days before the Jefferson-Jackson dinner in Iowa, an important stop on the campaign trail.

"As my family and I have worked through the grieving process, I've said all along what I've said time and again to others, it may be that the process closes the window on mounting a real campaign for president," he said. "I've concluded it has closed. There is no timetable for this process. The process doesn't respect or care for tings like filing deadlines or debates or primaries and caucuses. But I also know that I couldn't do this if the family wasn't ready."

Although he will not run, he pledged that he would "not be silent" and that he will "speak out clearly and forcefully so influence as much as I can where we stand as a party and where we need to go as a nation." Biden extolled the importance of preventing the "undo[ing] of the Obama legacy" in the upcoming election.

Biden delivered the news just minutes after calling reporters to the Rose Garden with no prior notice. His sister sister, Valerie, and close advisors were all in attendance in the audience as Biden stood next to his wife and President Obama.

He leaves behind a super PAC that had raised $3 million for his potential campaign, as well as 10% of the democratic electorate that supported him. Much of that support will likely now go to fellow Obama alum Hillary Clinton.

"We are so grateful for the gigantic outpouring of support from hundreds of thousands of Americans around the country in our effort to encourage the vice president to run. While the Vice President has decided not to run, we know that over the next year he will stand up for all Americans and articulate a vision for America's future that will leave no one behind," Draft Biden said in a statement following the announcement.

Biden, who will turn 73 in November, served as a senator for 36 years before being tapped as President Obama's running mate in 2008. Having been at Obama's side through the president's two terms, Biden had the greatest claim as the heir of Obama's legacy, a powerful asset when courting a constintuency of voters who elected Obama twice.

As Clinton's approval numbers faded in a cloud of scandal, many Democrats had been searching for a viable Plan B. Though Sen. Bernie Sanders' candidacy has attracted a strong grassroots following and he leads in the early primary state of New Hampshire, he has had trouble making inroads among black voters, a key Democratic voting bloc, and as a socialist, isn't seen as a plausible general election candidate among party elites. Biden would have been in a stronger position to tap large donors and build a national campaign infrastructure.

Draft Biden, the PAC that has encouraged, organized and fundraised for Biden as he deliberated his political future, was first formed in early 2015, but became more serious in May as Clinton found herself to be the focus of various political controversies.

Following the death of his son Beau Biden in June, the vice president has grieved publicly and has received an outpouring of sympathy that translated into numbers. Although not a candidate, he has consistently garnered between 15 percent and 20 percent of the vote.

But despite these encouraging signs, the vice president repeatedly claimed that he wasn't sure he had the "emotional energy" to run for office. As the other Democratic candidates in the field have raced around the country campaigning in advance of the first primaries, Biden has spent time with his family and laid relatively low.

Beginning in August, Biden embarked on a speaking tour in the South, where he met with top Democratic donors. These actions raised eyebrows among many voters, especially since Biden never denied that he was looking at entering the primary. But after meetings with senior advisors and family members, Biden left the primary to the four current candidates: Clinton, Sanders, Martin O'Malley and Lincoln Chafee.