What can possibly explain Joe Manchin and Kyrsten Sinema’s decision to sink voting rights protections? Why did they create a false narrative that the legislation had to be “bipartisan” when everyone, themselves included, knew bipartisanship was impossible?

Why did they say they couldn’t support changing Senate filibuster rules when only last month they voted for an exception to the filibuster that allowed debt ceiling legislation to pass with only Democratic votes?

Why did they co-sponsor voting rights legislation and then vote to kill the very same legislation? Why did Manchin vote for the “talking filibuster” in 2011 yet vote against it now?

Part of the answer to all these questions can be found in the giant wads of corporate cash flowing into their campaign coffers.

But if you want the whole answer, you need also to look at the single biggest factor affecting almost all national politicians I’ve dealt with: ego. Manchin’s and Sinema’s are now among the biggest.

Before February of last year, almost no one outside West Virginia had heard of Manchin and almost no one outside Arizona (and probably few within it) had ever heard of Sinema.

Now, they’re notorious. They’re Washington celebrities. Their photos grace every major news outlet in America.

This sort of attention is addictive. Once it seeps into the bloodstream, it becomes an all-consuming force. I’ve known politicians who have become permanently and irrevocably intoxicated.

I’m not talking simply about power, although that’s certainly part of it. I’m talking about narcissism – the primal force driving so much of modern America but whose essence is concentrated in certain places such as Wall Street, Hollywood and the United States Senate.

Once addicted, the pathologically narcissistic politician can become petty in the extreme, taking every slight as a deep personal insult.

I’m told Manchin asked Joe Biden’s staff not to blame him for the delay of Build Back Better and was then infuriated when Biden suggested Manchin bore some of the responsibility. I’m also told that if Biden wants to restart negotiations with Manchin on Build Back Better, he’s got to rename it because Manchin is so angry he won’t vote for anything going by that name.

The Senate is not the world’s greatest deliberative body but it is the world’s greatest stew of egos battling for attention. Every senator believes he or she has what it takes to be president. Most believe they’re far more competent than whoever occupies the Oval Office.

Yet out of 100 senators, only a handful are chosen for interviews on the Sunday talk shows and very few get a realistic shot at the presidency. The result is intense competition for attention.

Again and again, I’ve watched worthy legislation sink because particular senators didn’t feel they were getting enough credit, or enough personal attention from a president, or insufficient press attention, or unwanted press attention, or that another senator (sometimes from the same party) was getting too much attention.

Several people on the Hill who have watched Sinema at close range since she became a senator tell me she relished all the attention she got when she gave her very theatrical thumbs down to increasing the minimum wage, and since then has thrilled at her national celebrity as a spoiler.

Biden prides himself on having been a member of the senatorial “club” for many years before ascending to the presidency and argued during the 2020 campaign that this familiarity would give him an advantage in dealing with his former colleagues. But it may be working against him. Senators don’t want clubby familiarity from a president. They want a president to shine the national spotlight on them.

Some senators get so whacky in the national spotlight that they can’t function without it. Trump had that effect on Republicans. Before Trump, Lindsey Graham was almost a normal human being. Then Trump directed a huge amp of national attention Graham’s way, transmogrifying the senator into a bizarro creature who’d say anything Trump wanted to keep the attention coming.

Not all senators are egomaniacs, of course. Most lie on an ego spectrum ranging from mildly inflated to pathological.

Manchin and Sinema are near the extreme. Once they got a taste of the national spotlight, they couldn’t let go. They must have figured that the only way they could keep the spotlight focused on themselves was by threatening to do what they finally did last week: shafting American democracy.

• Robert Reich, a former US secretary of labor, is professor of public policy at the University of California at Berkeley and the author of Saving Capitalism: For the Many, Not the Few and The Common Good. His new book, The System: Who Rigged It, How We Fix It, is out now. He is a Guardian US columnist. His newsletter is at robertreich.substack.com