Sen. Bernie Sanders announced Wednesday that he is suspending his presidential campaign.

The big picture: It's an end to the campaign of the leading progressive in the race — and the candidate who seemed to be the clear front-runner for the Democratic nomination just a few months ago. It also makes Biden the presumptive Democratic nominee four months before the party's convention in Milwaukee.

Sanders, a democratic socialist whose signature campaign proposal was Medicare for All, had the enthusiastic backing of young voters and powerful momentum after a strong victory in the Nevada caucuses.

But he couldn't compete after one moderate candidate after another dropped out and endorsed Biden, driven by the desire to stop the nomination from going to a candidate they saw as too far left to defeat Trump.

Sanders has said he will support the Democratic nominee against Trump no matter what.

The backstory: Sanders gained a dedicated following in the 2016 primary, allowing him to enter the 2020 field with notable name recognition and strong grassroots support that helped fuel a massive fundraising operation.

He reached front-runner status near the turn of February, receiving the second-most delegates in Iowa before taking first in the New Hampshire primaries and Nevada caucuses.

But then Biden won South Carolina in February, largely due to his endorsement from "kingmaker" Rep. James Clyburn (D-S.C) and his strength with black voters.

That began his rapid comeback, fueled by the endorsements of two moderate candidates who dropped out just before Super Tuesday: former South Bend, Indiana, Mayor Pete Buttigieg and Sen. Amy Klobuchar.

Biden then dominated the Super Tuesday contests. He later won Michigan, the pivotal swing state that Sanders won in 2016, and Florida, extending what was effectively an insurmountable delegate lead.

After Super Tuesday, Sanders said he would remain in the race and debated Biden one-on-one in an effort to move the presumptive nominee to the left on several key issues. That strategy was at least partially successful, as Biden adopted Sanders' plan on college tuition and Sen. Elizabeth Warren's plan for bankruptcy reform.

The bottom line: The coronavirus has been smothering Sanders’ already difficult path to a comeback.

The Bernie movement was built on on massive rally crowds and huge canvassing efforts, all of which has come to a grinding halt with social distancing.

was built on on massive rally crowds and huge canvassing efforts, all of which has come to a grinding halt with social distancing. Sanders' chance to make his case on TV and social media has been crowded out by the public and media focus on the virus. For example, cable networks broke away from their primary coverage last month to discuss the virus.

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