Peggy Drexler is a research psychologist and the author of "Our Fathers, Ourselves: Daughters, Fathers, and the Changing American Family" and "Raising Boys Without Men." She is at work on a book about how women are conditioned to compete with one another. The opinions expressed in this commentary are hers. View more opinion on CNN.

The reason: Graham's allegiance to President Donald Trump and, specifically, his encouragement of investigations into her husband -- and Trump's Democratic rival -- Joe Biden, and her stepson, Hunter Biden. Trump and his allies have repeatedly made unfounded and false claims alleging that the Bidens acted corruptly in Ukraine.

Back in November, when Graham first suggested the Bidens would be investigated, Joe Biden expressed the same ire: "I am disappointed, and quite frankly I'm angered, by the fact -- he knows me; he knows my son; he knows there's nothing to this," the former vice president said of Graham, a Republican.

One bewildering aspect of this, according to Jill Biden, is that the Bidens and Graham, along with the late GOP Sen. John McCain, were "great friends" -- at least professionally. They traveled together, for politics. They had dinner together. "You know," said Jill Biden, "and now he's changed."

But, of course, so have many things in these times. If voting along party lines was a "sometimes" or even frequent occurrence back when Joe Biden was getting started as a senator, it's now, by most measures, an absolute.

Consider that Monday, after more than four months of investigation and prosecution -- including revelations last week of blockbuster evidence potentially on offer from former national security advisor John Bolton -- House Democrats concluded their impeachment efforts after only two Republican senators came forward to vote to hear from actual witnesses (like Bolton) in the President's trial.

And despite being impeached by the House, it looks like the President will officially be acquitted Wednesday.

What's more, isn't all fair in love and politics? Jill Biden's apparent hope or belief that the couple's friendship with Sen. Graham would trump his political ambitions -- or even just his political obligations -- is sweet, if true. And if true, certainly also naïve. After all, who among us hasn't experienced betrayal, or perceived betrayal, at the hand of a supposed friend? When you throw in politics, well, it is best to keep expectations low.

Indeed, the Bidens are lucky if this is their first -- even fifth -- encounter with disappointment in a friend's behavior, especially given their position and the high level of friends they keep.

If Graham and the Bidens' mutual friend John McCain were still alive, would Graham have flipped on Biden so readily, to benefit Trump? By many accounts, McCain knew how to be bipartisan, a bridge between two ever-disparate worlds. Graham considered him a personal hero: "There's no one I admired more than John McCain ... I loved John McCain ... I learned a lot from him; he's an American hero," Graham said last March in the aftermath of one particularly virulent McCain-bashing Twitter spree from Trump. Then, Graham said he would continue to try to help the President.

If McCain were still alive, would things be different? It's possible. It's possible that Graham would have thought harder about being as outspoken as he has been about the Bidens, had McCain (who, to put it mildly, had no love for Trump) been there to dissuade him from linking his fortunes to the President's. It's possible he would have stayed out of it entirely.

But Graham, after all, is not alone in shows of loyalty to the head of the Republican Party. Graham has conceded that his new allegiance to Trump has much to do with getting himself reelected in 2020. Indeed, the Republicans have held tight on this impeachment.

In fact, Graham would have been mainly alone, had he strayed from the course. That's a lot to ask of even a best friend. But the fact is that theirs -- Bidens' and Grahams' -- was perhaps an unusual alliance to begin with. Graham has always been very conservative. He has always been a politician. (So, it must be said, has Biden.)

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This doesn't mean Jill Biden is out of line for calling Graham out, especially in the wake of Graham's prediction Sunday on Fox News that the Senate will soon pursue testimony from Hunter Biden. Her apparent sense of hurt is, in fact, a normal human reaction -- even when found in the spouse of a politician.

It is a normal reaction in these abnormal, divided times -- so unusually fractious that (though we'll take Jill Biden's word for it) it's hard now to imagine the Bidens and Lindsey Graham ever hung out; it's all but impossible to recall a time when one might expect friendships to overrule political beliefs or ambitions.

These days, all is fair in love and politics. And it's unfair.