WASHINGTON — President Joe Biden was undergoing a routine medical checkup Thursday at Walter Reed National Military Medical Center, an event that is being keenly watched as the oldest president in history makes plans for an expected reelection campaign.

The president arrived at Walter Reed by helicopter and entered the hospital around 9:15 a.m. for what is likely an array of tests.

Biden, 80, last received a standard medical exam in November 2021. During that five-hour-plus visit, he went through a combination of blood, physical, gastrointestinal, dental, vision and neurological tests. Afterward, Dr. Kevin O’Connor, who has been Biden’s primary care physician since 2009, signed a six-page memo that called Biden “healthy, vigorous,” and deemed him “fit to successfully execute the duties of the Presidency.”

Many people, including Democrats, have expressed reservations about Biden seeking a second term in 2024. Only 37% of Democrats say Biden should pursue reelection, down from 52% prior to last year’s midterm elections, according to a recent poll from The Associated Press-NORC Center for Public Affairs Research. Overall, 22% of U.S. adults said he ought to run again.

Follow-up interviews with people polled in the survey indicated that Biden’s age was a major concern. People took note of his coughing, a greater sense of fragility in his walking and moments when he misspoke.

During Biden’s 2021 physical, a neurological exam found nerve damage to his hands and feet that had contributed to his stiffened gait. Spinal arthritis and compensation for a broken foot also caused him to walk more cautiously.

Biden underwent a colonoscopy in 2021, in which a 3 millimeter “benign-appearing polyp” was identified and removed.

Asked in a recent PBS interview about his age and ability to be president, Biden responded with what has become his stock line: “Watch me. It’s all I can say.”

Biden tested positive in July for COVID-19 and experienced “very mild symptoms,” according to the White House. The president continued to work even as he isolated himself in the White House.