A majority of Americans say Joe Biden was not mentally and physically capable of serving as president during his final year in office, according to new polling from Napolitan News Service.

The poll found 58% of American voters across party lines didn't think Biden was fit to run the country.

The polling of 1,000 registered voters across the U.S. was conducted Wednesday and Thursday, after it was revealed that Biden had been diagnosed with an advanced stage of prostate cancer that has already spread to his bones.

The poll also found 56% of respondents overall now think White House staff members –not Biden – were making the decisions during the president's final year in office, 2024.

There was a significant difference in responses between Republicans and Democrats.

On the question of whether Biden was physically and/or mentally capable of serving, 85% of Republicans didn't think so, while just 32% of Democrats didn't.

On whether staffers were making presidential decisions, 88% of Republicans thought so, compared to 24% for Democrats.

Napolitan News Service also asked voters about the former president's cancer diagnosis and whether he knew about it.

"Forty-six percent (46%) of voters believe President Biden likely knew he had cancer for several years while he was president," Napolitan said. "Thirty-nine percent (39%) disagree and believe that the cancer was just discovered."

Of those who say Biden likely knew he had cancer years ago, 93% say his closest aides also knew and covered it up.

The poll was conducted by RMG Research on May 20-21 and has a margin of error +/- 3.1 percentage points.