WASHINGTON (Reuters) - U.S. Senator Bernie Sanders passed a stress test this month after a heart attack required him to have two stents inserted in an artery in October, doctors to the Democratic presidential candidate said on Monday.

FILE PHOTO: U.S. Senator Bernie Sanders speaks during a campaign rally at Venice Beach in Los Angeles, California, U.S., December 21, 2019. REUTERS/Monica Almeida

Sanders’ campaign released letters from his main doctor and two heart specialists, who all gave the 78-year-old White House hopeful a clean bill of health after physical exams, electrocardiogram monitoring and a treadmill stress test.

“Mr Sanders is more than fit enough to pursue vigorous activities and an occupation that requires stamina and an ability to handle a great deal of stress,” said Philip A. Ades, the director of cardiac rehabilitation at the University of Vermont Medical Center (UVM), where the senator had the stress test on Dec. 11.

Ades said in his letter that Sanders’ exercise capacity was average for otherwise healthy men his age with no known heart disease.

Sanders was hospitalized after experiencing chest pains during a campaign event in Las Vegas on Oct. 1, and temporarily suspended campaign appearances.

He was back on the campaign trail promoting his agenda - which includes expansion of government-run healthcare - within weeks, and appeared healthy during televised debates in November and December.

The recovery, and the endorsements of high-profile progressives like Representative Alexandria Ocasio-Cortez, have seen Sanders’ polling numbers rise just as U.S. Senator Elizabeth Warren, who is also campaigning on left-wing proposals like a wealth tax, has seen her support dip.

Sanders has the support of about 18.9% of voters in the Democratic nominating contest, according to an average of recent opinion polls by Real Clear Politics. Sanders trails former Vice President Joe Biden, who is on 28.3%, and leads both Warren (15.3%) and South Bend, Indiana, Mayor Pete Buttigieg (8.3%) in the polling average.

Martin M. LeWinter, attending cardiologist at UVM, said in a letter that while Sanders had suffered modest heart muscle damage and was receiving some medication, he was not showing any negative symptoms.

“At this point, I see no reason he cannot continue campaigning without limitation and, should he be elected, I am confident he has the mental and physical stamina to fully undertake the rigors of the Presidency,” LeWinter wrote.