Biden's news, delivered in the Rose Garden at the White House, comes just after Clinton's strong debate performance which further boosted her standing in polls. Now, barring a disastrous performance at Thursday's House hearing on Benghazi, Clinton is on track to have the best month of her campaign.
After a summer of stumbles, the air of inevitability about Clinton's candidacy is making a comeback.
"Her campaign has already hit rock bottom. With the debate and after this announcement and then the Benghazi hearings, we are starting to see excitement and momentum build. She is peaking when she should," said Bakari Sellers, a CNN contributor and former South Carolina lawmaker, who has endorsed Clinton.
Since last week's debate, Clinton has solidified her support among Democrats, leading Bernie Sanders by more than 15 points in the latest CNN/ORC poll
that included Biden as a potential candidate. Without Biden as an option, Clinton leads by 23%
"You can see from the polling numbers that have come out this week how that has certainly reassured and solidified her support among Democrats across the country," said Sen. Claire McCaskill, D-Missouri. "I think this will push her even further in that direction."
The possibility of a Biden campaign threatened to derail Clinton's strength with the Obama coalition, particularly among African-American voters. Early state polls in South Carolina showed Biden chipping away at Clinton's support, yet without Biden in the race her support jumps from 59% of black voters to 84%, according to a CNN/ORC poll. Sanders, who stood to gain from a Biden entry, has yet to consistently break 30% in national polls and only gets 7% of the black vote in South Carolina.
"There was a real framework for Joe Biden," Sellers said. "The race would have been competitive."
South Carolina—and the entire South—was poised to be a fierce battleground between Clinton and Biden, but in recent days Clinton flashed her strengths with an eye toward Biden. On Saturday, she nabbed the endorsement of the Alabama Democratic Conference. And on Tuesday, her campaign released a list of 50 endorsements from African-American mayors—more than half came from the Palmetto State.
Then, Rep. Jim Clyburn, a leading figure in the Congressional Black Caucus, said that Biden couldn't create a path.
"If I were advising him, I would not advise him to get in," Clyburn, 75, said in an interview with The Huffington Post. "In my opinion, he would not do himself any favors by getting in."
But, even as he declared he wouldn't be a candidate citing the grief over the recent loss of his son Beau, Biden made it clear that he saw himself as a power center and possible thorn in Clinton's side.
"I don't think we should look at Republicans as our enemies," he said, tweaking Clinton for a comment she made at the first Democratic debate. "They are our opposition; they're not our enemies. And for the sake of the country, we have to work together."
Sanders, echoing Biden in later comments, also knocked Clinton, saying he "would not use the word, quote, end quote, enemies, to describe fellow Americans." Both Sanders and Biden have a history of partisan attacks aimed at Republicans, yet see some benefit in seeming to take the high road vis-a-vis Clinton.
For his part, energized by the buzz of his would-be candidacy, Biden said he wouldn't be silent and positioned himself as a vocal defender of Obama's record, offering candidates a playbook.
"This party, our nation will be making a tragic mistake if we walk away or attempt to undo the Obama legacy," Biden said Wednesday. "The American people have worked too hard and we've come too far for that. Democrats should not only defend this record and protect this record, they should run on the record."
Clinton extended an olive branch to Biden in a statement after his announcement -- and responded directly to his comments regarding the current White House.
"Joe Biden is a good man and a great Vice President," Clinton said. "Like millions of others, I admire his devotion to family, his grace in grief, his grit and determination on behalf of the middle class, and his unyielding faith in America's promise.
"As Vice President, Joe has been by President Obama's side for every pivotal decision," she added. "He helped save the auto industry and pull our economy back from the brink of depression. He continues to fight for higher wages, safer communities, and a more peaceful world. It's a record to be proud of, defend, and build on."
She continued, "And I am confident that history isn't finished with Joe Biden. 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."
Among Democratic candidates, there is largely agreement on Obama's record, though Clinton has appeared more hawkish and split with the president on the Trans Pacific Partnership, a move that was cheered by big labor.
Her position put her on the same side as Sanders, and would have forced Biden into an uncomfortable spot: He could support the Obama administration or side with his allies in labor unions, whose support he'd badly need in the 2016 race -- but not both.
David Axelrod, however, no doubt recalling Clinton's early stumbles in 2008 and then her strong showing once she faltered, sounded a note of caution about the run of good news for Clinton.
"With polls looking up, Q is how @HillaryClinton deals with good news?" he tweeted. "She's generally been better w/ her back to wall than as front runner."