Sanders was at a campaign event on Tuesday in Vegas when he began experiencing chest pains. He was taken either to a doctor's office or a hospital -- although it would seem a hospital is far more likely -- and tests revealed he had a blockage in one of his arteries.

Surgeons inserted 2 stents to clear the blockage. The 78-year-old Senator is resting in the hospital and campaign events over the next few days have been canceled.

Sander's senior adviser, Jeff Weaver, said, "We are canceling his events and appearances until further notice, and we will continue to provide appropriate updates."

Sanders is the oldest of the remaining Democratic candidates ... Joe Biden is 76. BTW ... Donald Trump is 73.

Sanders has been struggling in the polls ... overtaken in a big way by fellow Senator Elizabeth Warren.