Sen. Bernie Sanders on Friday pushed back against attacks on his age and invited voters in the 2020 Democratic presidential primary to see his consistent record of promoting progressive causes and advocating for the working class as an asset.

"Having a long record gives people the understanding that these ideas that I am talking about—they are in my guts," Sanders said. "They are in my heart."

"This is who I am as a human being," the senator added, "and it ain't gonna change."

The Vermont senator, 78, is one of three frontrunners in the primary race. The other two top tier candidates are former Vice President Joe Biden, 76, and Sen. Elizabeth Warren (D-Mass.), 70. 

While age has long been a simmering issue on the trail, Sanders became the target of focused attacks this month when a minor heart attack resulted in surgery to place two stents into a blocked artery on October 4. But despite a flurry of media handwringing over his durability, the senator appeared in good health less than two weeks later onstage for a CNN/New York Times debate on October 15, where his performance was favorably received.

Days later, on October 19, Sanders held a rally in Queens where Rep. Alexandria Ocasio-Cortez (D-N.Y.) and other New York politicians announced their endorsement of his run for president in front of a crowd of over 26,000 people. 

Sanders' comments Friday came in response to a reporter asking about the viability of the campaign in light of the health scare after the senator reportedly brought his age up unprompted in a town hall Thursday and used his long record of fighting for progressive causes to draw a distinction between his campaign and those of his rivals. 

"What distinguishes my candidacy from the others is I have been fighting for the working families of this country for many, many decades," said Sanders.

One group that appears unconcerned with Sanders' age is younger voters, who back the senator by large margins. As Common Dreams reported Tuesday, the senator draws support from 45 percent of voters aged 18-29. The next highest level of support is for Warren, at 17 percent.

"No other candidate running has been so resilient," tweeted journalist Walker Bragman Tuesday in response to the polling numbers. "Bernie holds the largest rallies, has by far the largest donor network, excites young people, and has been shown to perform best against Trump in states like Texas and Wisconsin."

That consistency, Sanders said Friday, is central to his candidacy. 

"The ideas that I am fighting for now didn't come to me yesterday," said Sanders.