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 and has raised over $25m to £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”.
Polls show Mr Sanders is third in the Democratic race behind Massachusetts Senator Elizabeth Warren and former vice-president Joe Biden.