Vice President Joe Biden announced today that he will not run for president in 2016.

Biden said he had concluded that the window for mounting a realistic presidential campaign had closed. He made the announcement in the White House Rose Garden, with President Obama by his side.

In a speech that often sounded like it could have been a campaign announcement — and perhaps was written for that purpose — Biden laid out his views on the political scene. He said he wanted to fight for the middle class, that there was too much big money distorting American politics, and that he wanted to push to find a cure for cancer, because "it's possible."

And though he didn't name Hillary Clinton, he took another shot at her. Clinton had said in the first Democratic debate that "the Republicans" were "probably" the enemy she was most proud of making during her career. In Biden's speech, he repeated a point he's made several times in the past few days — saying that Republicans shouldn't be treated as "enemies," and saying that they were instead "our opposition."

Biden's announcement makes clear that Clinton is the only establishment candidate

So far in this campaign, frontrunner Hillary Clinton's main threat has been a challenge from the left from Sen. Bernie Sanders. Sanders has proved more formidable than many expected, but many in the party believe his effort is ultimately doomed because Democratic voters will be reluctant to make a "democratic socialist" their nominee.

Other than Sanders, Clinton is facing only nominal opposition. She had deterred every single ambitious and longtime Democrat from running except for former Maryland Gov. Martin O'Malley, whose campaign has utterly failed to catch on. And though Jim Webb was initially viewed by some as a potential threat, he quit the Democratic race on October 20. So if something went badly wrong with Clinton's campaign, she could still be confident that she was likely to emerge on top.

Biden's entry into the race could have changed this. Though he faced a huge number of obstacles and would have started as an underdog, he at least was well-known to the Democratic electorate as well as the party's elites, and has the sort of résumé that past nominees have had. He would have been well-positioned as the insiders' second choice, should Clinton have lost support.

But there was little enthusiasm among voters or Democratic elites for a Biden candidacy

Yet despite the fact that the sitting vice president is seemingly a natural successor to President Obama, there's been little encouragement for a Biden campaign from Democratic elites, interest groups, or voters. Progressive activists are more excited about Bernie Sanders, while the party's mainstream has overwhelmingly fallen behind Hillary Clinton.

Meanwhile, Biden didn't seem to have an obvious policy-based case for why liberals should prefer him to Clinton, and his election wouldn't have meant a demographic first like hers would. He's culturally out of step with today's Democratic Party. And his very persona has made him difficult to take seriously — his jokey, unscripted, gaffe-prone style makes him a figure of fun to many.

Accordingly, he's been doing quite badly in the polls — according to the HuffPost Pollster averages, he trails Clinton by 30 points nationally and in Iowa, and by 20 points in New Hampshire (where Sanders has led many recent polls). This has been the case both before and after Clinton officially announced her candidacy. Though Biden's favorability ratings looked quite good this summer, it's unclear how long that would have lasted had he entered the partisan campaign fray.

Furthermore, it didn't look like Biden had a good opportunity in any early state. His allies had floated a strategy in which he'd focus on South Carolina, but he's doing just as badly in polls there, and the state's most respected Democrat, Rep. Jim Clyburn, said recently that Biden shouldn't run. And Biden is far, far behind both Clinton and Sanders in raising money and building a campaign organization.

Still, there was some nervousness in the party about Clinton's declining poll numbers — she now regularly trails Republicans in general election matchups — and her email scandal. So even if few party members would have endorsed Biden, some may have been quietly grateful that there was a potential alternative to Clinton in the race.

Now, though, that's not the case. It looks like Clinton, Sanders, or bust.

WATCH: Joe Biden laugh off a lie