Bernie Sanders wouldn't let a heart attack stop him. In the end, an unseen enemy provided the final blow to his presidential campaign.

The Vermont senator on Wednesday cited the coronavirus pandemic sweeping across the globe as a decisive factor in dropping out the race for the Democratic presidential nomination. He conceded that his bid to defeat former Vice President Joe Biden in the primaries was "virtually impossible."

"I cannot in good conscience continue to mount a campaign that cannot win and which would interfere with the important work required of all of us in this difficult hour," Sanders said in a livestream from his Vermont home.

Though he suspended his presidential campaign, Sanders said he would remain on primary ballots so he could win more delegates and press his agenda at Democratic National Convention in August.

"Let us go forward together, the struggle continues," the democratic socialist told his supporters.

During the campaign, he promised a drastic political and economic overhaul, including implementing a single-payer "Medicare for All" health-care system, canceling all student debt and taxing the wealthy at higher rates.

"It has a Leninist feel to it," CNBC commentator Jim Cramer said in October about Sanders' agenda.

Just last fall, the 78-year-old Vermont independent abruptly canceled all appearances while campaigning in Nevada. In a statement on Oct. 2, campaign advisor Jeff Weaver said Sanders "experienced some chest discomfort" during an event the previous night. Testing found a "blockage in one artery," and Sanders had two stents inserted, Weaver said.

Days later, the campaign acknowledged that Sanders "was diagnosed with a myocardial infarction" — a heart attack. In two more weeks, Sanders made his first public appearance after the hospitalization — robustly taking part in the fourth Democratic debate of the 2020 cycle. At the debate, he thanked well-wishers "for their love, for their prayers."

"I'm healthy, I'm feeling great," Sanders said to a round of applause.

"We are going to be mounting a vigorous campaign all over this country," he promised. "That is how, I think, I can reassure the American people."

Sanders, who is serving his third term in the Senate, was the oldest candidate seeking to challenge President Donald Trump in November. The Vermont independent is a year older than Biden and five years older than Trump, who is the oldest person elected to a first term in the White House.

Cerebral, forceful, focused, unbending and rumpled, the fire-spewing Sanders twice ran for the Democratic presidential nomination even while refusing to join the party. He is registered as an independent, although he caucuses with the Democrats in the Senate.

In the 2016 presidential race, Hillary Clinton had been considered a no-brainer to win the party nomination, but the upstart Sanders grabbed a shocking 23 primaries and caucuses plus pledges of support from more than 4 in 10 delegates to the Democratic National Convention, but still falling short.

"I look forward in the coming weeks to continue discussion between the two campaigns to make certain that your voices are heard and that the Democratic Party passes the most progressive platform in its history, and that Democrats actually fight for that agenda," Sanders said in acknowledging defeat to Clinton. "I also look forward to working with Secretary Clinton to transform the Democratic Party, so that it becomes a party of working people and young people, and not just wealthy campaign contributors, a party that has the guts to take on Wall Street, the pharmaceutical industry, the fossil fuel industry and the other powerful special interests that dominate so much of our political and economic life."

In the midst of this year's coronavirus crisis, Sanders called for a massive response that "protects the interests of all our people regardless of their income, or where they live."

"In other words, this is not just about giving tax breaks to large corporations, but about remembering the people today who don't have much money, who are nervous about their economic futures and health-care prospects," he said last month.