Bernie Sanders, the Democrat presidential candidate, has been hospitalised and had two stents inserted for a blocked artery.
The 78-year-old US senator’s campaign team canceled his events and appearances "until further notice."
Mr Sanders experienced chest discomfort during an event on Tuesday and sought medical attention.
The campaign said two stents were "successfully inserted" and that Mr Sanders "is conversing and in good spirits."
He has no history of heart problems.
Jeff Weaver, his senior adviser, said: "During a campaign event yesterday evening, Senator 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.
"Senator Sanders is conversing and in good spirits. He will be resting up over the next few days. We are canceling his events and appearances until further notice, and we will continue to provide appropriate updates."
US election runners: Bernie Sanders
It came only four months ahead of the Iowa caucuses, in which the first state will vote to choose the Democrat presidential nominee.
Mr Sanders has a relentless campaign schedule, sometimes speaking at five events in a day.
On Tuesday his campaign announced a huge fundraising total for the third quarter of 2019, with more than $25 million.
His campaign has received donations from more then one million people.
Faiz Shakir, his campaign manager, said the "state of the campaign is strong."
Mr Shakir said: "Bernie is proud to be the only candidate running to defeat Donald Trump who is 100 per cent funded by grassroots donations.
"Media elites and professional pundits have tried repeatedly to dismiss this campaign, and yet working-class Americans keep saying loudly and clearly that they want a political revolution."