Barack Obama's former doctor is worried about the advancing age of Joe Biden, whom he said looked "frail" during the Democratic debates when confronted by Kamala Harris about his history with race relations.

"Biden made me nervous," said Dr. David Scheiner, who was Obama's personal physician for the 22 years before he became president. "Harris started attacking him and he looked frail to me. I sort of got the feeling he wasn’t very strong. It was similar to the feeling I got when Republicans started attacking [special counsel Robert] Mueller so fiercely."

But despite his concerns, Scheiner would still vote for Biden, or any Democrat in the crowded primary field, over President Trump. He called the president "incredibly dangerous" and said Trump, 73, was exhibiting his own health and aging problems.

He noted that Trump is overweight, has a poor diet and has a sedentary lifestyle, which puts him at risk of cardiovascular problems. He has also been concerned about the president's speech patterns and worries about his risk for dementia and stroke.

"He garbles. That is a really critical issue. This is not trivial," Scheiner said. "Seth Myers jokes about it but it's not a joke. The guy just can't find the words. He's having difficulty."

The Washington Examiner interviewed Scheiner on Thursday about a recent white paper that concluded nearly all candidates running for president had a high probability of surviving four years past inauguration day. Scheiner reiterated his stance that voters should not accept a candidate’s declaration of health at face value, and said they should undergo an independent medical evaluation. Older presidents, he said, should undergo an MRI of the brain to check for strokes, and should provide details of their health to the public.

Dr. David Scheiner. (Screenshot/CNN)

Presidential candidates since Ronald Reagan have released some of their medical records to the public, but no law demands their release and no candidate has disclosed a full medical history.

"It’s too important to allow someone to get in there about whose health we know nothing," Scheiner said of the presidency.

He has been concerned about the issue not only because Trump has either disclosed little medical information, or the information he has disclosed has been cast in doubt, but also because several Democrats running for president would be over age 70 on Inauguration Day.

There are simply changes to the brain that happen with aging that affect how people speak and how they approach situations, he said. While he allowed that people more advanced in age could offer wisdom and experience, he said he sees older people react to situations with less creativity.

For instance, he said while Bernie Sanders was "energetic," he was still running the same campaign he did in 2016. Despite Sanders being one of the oldest candidates in the race, however, he enjoys a strong following among young people.

"His agenda is such a revolutionary thing," Scheiner said. "He’s sort of for revolution and they support that."

The one candidate who might be an exception to his observations about aging, Scheiner said, might be Sen. Elizabeth Warren of Massachusetts, 70, whom he said had creative policy approaches and detailed plans, though might not be likeable enough to win the White House.

"She's incredibly bright and sharp on these things," he said. "But even on her I would still want a [medical] test to be done."

Even he is not immune to aging, he admitted.

"I'm not the doctor I was 20 years ago," said Scheiner, who will turn 81 in September.

Scheiner now works at a free clinic. When Obama was a candidate for the presidency he vouched for his "excellent health," but the two are not still connected after Scheiner came out against the Affordable Care Act, Obama's signature healthcare law, in favor of a healthcare system in which the government would become the only payer of healthcare services.

But there is one candidate in the primary who reminds him of his former patient, and that's South Bend, Indiana, Mayor Pete Buttigieg. While he did worry that Buttigieg, at 37, lacked experience, he called him "brilliant" and "just wonderful."

"He can differ with you politically but he doesn’t insult you," Scheiner said of Buttigieg's similarities to Obama. "He doesn't create enmity."