The Democratic candidate for 2020, Bernie Sanders, 78, has canceled election campaigns after surgery until further notice.
Mr. Sanders was treated at the hospital for arterial blockage after experiencing chest pain at a Nevada event on Tuesday.  An advisor to the Senator from Vermont said he was in a "good mood" and would rest in the coming days.
Mr. Sanders had previously announced that his campaign had raised more than $ 25 million (£ 20 million).
Senior Advisor Jeff Weaver said in a statement, "After a medical examination and examination, it was found that he had a blockage in an artery and two stents were successfully introduced."
Mr. Weaver said Mr. Sanders was "entertained and in a good mood" and will "rest in the coming days".
A stent is a small mesh tube designed to keep the arteries open. Obtaining stents is "a minimally invasive procedure," usually with a short recovery time, according to the US National Heart, Lung and Blood Institute.
It is unclear how long Mr. Sanders will recover from his operation and whether this will affect his appearance in the next democratic debate on 1
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Surveys show that Mr. Sanders is third in the Democratic race behind Massachusetts Senator Elizabeth Warren and former Vice President Joe Biden The largest Democratic candidate in a quarter so far.
If Mr. Sanders won the presidency, he would be the oldest in the US to hold the office.
Mr Sanders had been to Las Vegas to attend It was joined by some other 2020 candidates on Wednesday for a weapons security forum.
Many of his rivals from the president wished him a "speedy recovery."
Mr. Sanders announced on Tuesday a purchase of $ 1.3 million television advertising in Iowa, but in the US media reports that Wednesday began his campaign to cancel the ads. It is unclear why. In Iowa, the first election campaign of the US Presidential Contest takes place. Senator's health also hit the headlines last month when he canceled three events in South Carolina after losing his vote and took two days to recover.
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Mr. Sanders calls himself a Democratic Socialist
He is the longest-serving Independent in the history of Congress, but he fights for the nomination of Democrats, as he says. As a third-party candidate, he would reduce his chances of winning the presidency.
When he applied for the Democratic nomination in 2016, he was Hillary Clinton's closest rival.
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Its 2020 platform has largely focused on its universal health plan, Medicare for All. Politics has also become a central issue for Democrats in recent debates, with moderates such as Mr Biden criticizing them as unworkable and overpriced.