The 2020 Democratic presidential candidate Bernie Sanders, 78, has been in hospital to receive two stents for an arterial blockage.
He was admitted after experiencing chest pain during a campaign event on Tuesday evening in Las Vegas, Nevada.
A statement from his senior advisor said the Sanders campaign has cancelled all upcoming "events and appearances until further notice".
Mr Sanders had earlier announced his campaign has raised over $25m (£20m).
The Vermont senator's senior adviser Jeff Weaver said in a statement on Wednesday: "Following medical evaluation and testing he was found to have a blockage in one artery and two stents were successfully inserted."
He said Mr Sanders is "conversing and in good spirits" and will be "resting up over the next few days".