Sen. Bernie Sanders, 78, was hospitalized Tuesday night in Las Vegas.
"During a campaign event yesterday evening, Sen. Sanders experienced some chest discomfort. Following medical evaluation and testing he was found to have a blockage in one artery and two stents were successfully inserted," said Sanders’ senior adviser Jeff Weaver on Wednesday in a statement.
"Sen. Sanders is conversing and in good spirits. He will be resting up over the next few days," Weaver said. "We are canceling his events and appearances until further notice, and we will continue to provide appropriate updates."
Following the Tuesday morning statement from the campaign, Sanders tweeted later in the day that he's "feeling good" and, on-message, used the opportunity to plug Medicare for All.
Sanders remains hospitalized, a source told ABC News.
Weaver led an all-staff phone call this morning to inform the entire campaign, according to a source familiar with the call. An additional source familiar described the call as subdued.
ABC News first learned from officials about the Vermont senator being hospitalized after his plans to attend a presidential gun forum changed.
He is one of three septuagenarians in the Democratic primary, and the oldest. Former Vice President Joe Biden is 76. Massachusetts Sen. Elizabeth Warren is 70.
President Donald Trump, who was the oldest elected president, is 73. If elected, Sanders would be the oldest president to serve.
Though Sanders has yet to release medical records this cycle, he told reporters in September that he "absolutely" would do so before the primaries.
"You know, I think it’s the right thing to do," he said. "The American people have the right to know about whether the person they are going to be voting for for president is healthy, and we will certainly release our medical records before the primaries.”
Sanders' last public appearance was at the Las Vegas shooting memorial on Tuesday night. Earlier this week, he was in New Hampshire where he held seven back-to-back events over the course of two days.
The White House hopeful was expected to travel to California later this week. The campaign has not specified how long Sanders will be sidelined, but ABC News confirmed that at least two of his planned events Thursday, a town hall at Cal State University, Bakersfield and a rally at Fresno City College, have been canceled, according to the schools.
There was an immediate outpouring of support for Sanders following his campaign's announcement that he has been hospitalized as fellow lawmakers – including his opponents for the Democratic presidential nomination – have put politics aside to wish him a full and speedy recovery.
Biden, a fellow 2020 Democratic presidential front-runner, was among those who tweeted words of support. At a campaign event in Las Vegas on Wednesday, Warren said she's "called" and "texted" to send her best wishes.
On Wednesday, Sanders was on a campaign swing through Nevada with plans to host a town hall on Medicare for All and social security in Las Vegas ahead of his scheduled appearance at the Giffords and March for Our Lives’ Presidential Gun Safety Forum on Wednesday. Giffords is an organization led by former Rep. Gabrielle Giffords, D-Ariz., who became a strong advocate for gun control in the aftermath of a failed 2011 assassination attempt on her life at a political event.
The news of Sanders' hospitalization comes on the heels of the presidential contender announcing he raised a whopping $25.3 million in the third quarter – the largest quarter for any Democratic candidate this year, and a number that, on its own, eclipses Warren's entire first and second quarters' total.
Sanders entered his second bid for president, after falling short to Hillary Clinton in the 2016 Democratic primary, as a known quantity.
Despite the presence of another liberal stalwart in the 2020 race, Warren, the former mayor of Burlington continues to carve out a lane for himself in a crowded field that still counts 19 with his blunt style and progressive ideals - including his signature Medicare for All proposal, which has been embraced by several of his Democratic rivals and often a key source of debate throughout the protracted contest.
Last month, Sanders told ABC's "The View" that 2020 is different because "many of the ideas that I talked about," including universal health care, "were perceived to be radical," but now "a lot of candidates and the majority of the American people agree."
This is a developing story. Please check back for updates.
ABC News' Adam Kelsey contributed to this report.