Losing It
By 
According to Jane Mayer, Dianne Feinstein chose to seek another term, at age 85, apparently despite being in at least the early stages of dementia:
It was a good question. Feinstein seemed sharp and focussed. For decades, she has been the epitome of a female trailblazer in Washington, always hyper-prepared. But this time, after Dorsey responded, Feinstein asked him the same question again, reading it word for word, along with the Trump tweet. Her inflection was eerily identical. Feinstein looked and sounded just as authoritative, seemingly registering no awareness that she was repeating herself verbatim. Dorsey graciously answered the question all over again.
[…]
But many others familiar with Feinstein’s situation describe her as seriously struggling, and say it has been evident for several years. Speaking on background, and with respect for her accomplished career, they say her short-term memory has grown so poor that she often forgets she has been briefed on a topic, accusing her staff of failing to do so just after they have. They describe Feinstein as forgetting what she has said and getting upset when she can’t keep up. One aide to another senator described what he called a “Kabuki” meeting in which Feinstein’s staff tried to steer her through a proposed piece of legislation that she protested was “just words” which “make no sense.” Feinstein’s staff has said that sometimes she seems herself, and other times unreachable. “The staff is in such a bad position,” a former Senate aide who still has business in Congress said. “They have to defend her and make her seem normal.”
After her absolutely disastrous performance at the Coney Barrett hearings — which, as Mayer observes, left Barrett more popular than at the beginning despite the extraordinary nature of the midnight confirmation — Schumer had The Talk with her…and then had to have it again because she forgot about it!
Schumer had several serious and painful talks with Feinstein, according to well-informed sources. Overtures were also made to enlist the help of Feinstein’s husband, Richard Blum. Feinstein, meanwhile, was surprised and upset by Schumer’s message. He had wanted her to step aside on her own terms, with her dignity intact, but “she wasn’t really all that aware of the extent to which she’d been compromised,” one well-informed Senate source told me. “It was hurtful and distressing to have it pointed out.” Compounding the problem, Feinstein seemed to forget about the conversations soon after they talked, so Schumer had to confront her again. “It was like Groundhog Day, but with the pain fresh each time.” Anyone who has tried to take the car keys away from an elderly relative knows how hard it can be, he said, adding that, in this case, “It wasn’t just about a car. It was about the U.S. Senate.”
Feinstein basically knifed every marginal Democratic Senate candidate in the back while doing a job she was no longer able to do in service of her ego. There would be no possible defense for her decision to run again at age 85 even if she wasn’t also a sub-replacement-level senator wasting a safe seat. And the defenses being offered are just pathetic:
Some former Feinstein aides insist that rumors of her cognitive decline have been exaggerated, and that video clips taken out of context can make almost anyone look foolish. They also bridle at singling out her condition, because declining male senators, including Strom Thurmond, of South Carolina, and Robert Byrd, of West Virginia, were widely known by the end of their careers to be non-compos mentis. “For his last ten years, Strom Thurmond didn’t know if he was on foot or on horseback,” one former Senate aide told me. The former Senate Majority Leader Harry Reid, of Nevada, is said to have snapped at a staffer who claimed to be relaying what Byrd thought. “Knock it off,” Reid supposedly said. “Everyone knows it’s what you think.” In contrast, one former aide to Feinstein argues that, even if her faculties are diminished, “she’s still smarter and quicker than at least a third of the other members.”
When you’re invoking Strom Thurmond as the defense against charges that you don’t know when to quit you’re just implicitly conceding she should have left at least two terms ago.