The fact that Joe Biden was declining mentally was no secret to anyone who saw him walking or talking or interacting with people during the last part of his presidency.

After all, he already had been described by a special counsel who gave him a pass for violating federal law regarding government documents as a senior citizen with a failing memory.

But just how bad it got will surprise many: He “needed fluorescent tape fixed to the carpet to show him where to walk at events.”

That’s according to a report assembled by the Daily Mail.

“That way the octogenarian would know where to go without having someone to guide him,” it explained.

And when he was preparing for his now infamous debate with Donald Trump, he “seemed to think he was ‘President of NATO’ rather than the United States.”

The report cites several books, probably at least four, that are being prepared about his term in the White House.

In a book planned by author Chris Whipple, Ron Klain, Biden’s chief of staff for two years and adviser for the 2024 campaign, talked about preparing for the debate that ended Biden’s career.

“We sat around the table. [Biden] had answers on cards, and he was just extremely exhausted. And I was struck by how out of touch with American politics he was. He was just very, very focused on his interactions with NATO leaders. He just became very enraptured with being the head of NATO.”

Whipple noted Klain “wondered half-seriously if Biden thought he was president of NATO instead of the U.S.”

The tape was needed in New Jersey Gov. Phil Murphy’s house so that Biden “wouldn’t get lost” after the debate, the report said.

Tape on stages to mark locations is routine for politicians, but “Biden needed the trail of bright markings to serve as ‘colorful bread crumbs [that] showed the leader of the free world where to walk,’” it explained.

Authors Jonathan Allen and Arnie Parnes have another planned book, and explained Biden would know “to look for that.”

His verbal and mental flubs already are legend, with a book detailing the multitudes, including when he told an audience member to stand up and let others see him, even though the person was in a wheelchair.

He also called, during news conferences, for dead people to be noticed.

Further, Josh Dawsey, Tyler Pager and Isaac Arnsdorf have yet another book coming out, and though Biden gave no interviews to the writers, more details are expected to be revealed.

Klain told Whipple that during preparations for the debate, Biden lasted only 45 minutes during a scheduled 90-minute mock debate, and didn’t seem to grasp the topics.

The second mock debate ended even more quickly: “I’m just too tired to continue and I’m afraid of losing my voice here and I feel bad. I just need some sleep. I’ll be fine tomorrow,” Biden said.

Prominent Democrats defended Biden’s capabilities up to the point of the debate and for a few, even after.

In a report at Fox News, Biden campaign aide Ashley Allison charged she felt “lied to” over what she had been told about Biden’s status.

“I think I hadn’t been around the president before that debate, and I worked for Joe Biden. And if the people around him knew that he was not capable, it is unacceptable to me that they allowed him to go onto that stage. I deserve better as a voter, not even as a Democrat, as a voter and as an American, I do.”