In February 2018, I went to Arizona to report on what progressive Democrats there thought about Kyrsten Sinema, the centrist whose Senate vote is key in the fight over President Biden’s agenda and political prospects.

At the time, Ms. Sinema, then a three-term congresswoman, was the leading Democrat for the state’s Senate seat being vacated by Jeff Flake, a Republican. She was handpicked by Chuck Schumer, the Senate Democratic leader, for her fund-raising acumen and a carefully curated moderate image that was believed to play well in Arizona, which hadn’t elected a Democrat to the Senate in 30 years.

Before flying to Phoenix, I asked Ms. Sinema’s aides whether she would be doing any campaigning or whether I could come see her — it was a week of congressional recess when members of Congress tend to spend time in their districts.

After obfuscating about her whereabouts, Ms. Sinema’s team finally told me to meet her at a bookstore in Phoenix for what was described as a round-table discussion with local businesswomen. When I got there, I encountered a highly unusual scene for a major campaign.

There was nobody else at the event, just the seven businesswomen, Ms. Sinema and her highly attentive staff (one aide unwrapped a straw before carefully placing it in Ms. Sinema’s can of La Croix), me and a small CNN crew.

She spent the 38-minute discussion — seemingly conducted purely for the benefit of The Wall Street Journal, where I worked at the time, and CNN — taking every opportunity to praise President Donald J. Trump and her meetings with him. When she was asked about child care, she said Mr. Trump’s daughter Ivanka Trump was working on it.

In our subsequent 13-minute interview, Ms. Sinema couldn’t name any topics in which she disagreed with Mr. Trump. When I asked what her younger self, who worked for Ralph Nader’s presidential campaign, would think of her in 2018, she said she would be “proud of the growth.”

And she wouldn’t say whether she had given up on her former, more liberal beliefs, but she stressed that she had prioritized results over rhetoric.

“What I’ve learned to do is use the tools and skills that I’ve learned to be productive and get stuff done,” she told me. “Getting stuff done is amazing. It’s amazing when you can say, ‘I’ve delivered real results.’”

What is perhaps most notable about that interview is that she did it at all. Ms. Sinema rarely granted requests for sit-down interviews with national reporters during the rest of her 2018 campaign. Since coming to Washington, she has been one of the most elusive senators on Capitol Hill.

She doesn’t engage with Washington reporters in a serious way, doesn’t hold open-to-the-public events in Arizona and has effectively cut off communication with the local progressive groups that worked to get her elected in 2018. Her spokesman did not respond when I emailed him.

Ian Danley, the executive director of Arizona Wins, a coalition of 32 progressive advocacy organizations, said his group had registered nearly 200,000 new voters and knocked on more than two million doors in support of Ms. Sinema’s 2018 campaign. She has not once met with his group or its partners since taking office in 2019, he said.

That, Mr. Danley said, prompted the frustration that led to the viral ambushing of Ms. Sinema over the weekend in a bathroom at Arizona State University, where she teaches classes on social work and fund-raising. Activists from Living United for Change in Arizona, one of the groups in the Arizona Wins coalition, pressed Ms. Sinema to support the $3.5 billion Democratic legislation that would expand the social safety net.

“What’s she supposed to do, she asked for a meeting — they tried to go meet with the staff and the senator, that doesn’t happen,” Mr. Danley said. “That’s a breakdown of constituent services, a breakdown of leadership — that’s not the fault of young people who are trying to lobby and influence their elected officials.”

Ms. Sinema, in a blistering statement, called the bathroom episode “not legitimate protest.”

Another activist tried without success to engage Ms. Sinema on her flight to Washington from Phoenix on Monday and there was another group waiting for her at Reagan National Airport. There, she pantomimed listening to something on her iPhone, which was odd because during the flight she had her AirPods in.

What happens next with Ms. Sinema is anyone’s guess. Unlike Senator Joe Manchin III of West Virginia, her fellow Democratic holdout on Mr. Biden’s legislation, Ms. Sinema hasn’t publicly articulated what she wants from the negotiations, a development that got her skewered on the latest episode of “Saturday Night Live.”

Perhaps the thing to know about Ms. Sinema is how she views her own political metamorphosis. After beginning her career so far on the liberal end of politics that she refused to take campaign contributions (“that’s bribery,” she said while running for the Phoenix City Council in 2001) and wrote letters to the Arizona Republic condemning the very idea of capitalism, Ms. Sinema has gone to great lengths to define herself as the opposite of what she was before.

“When I was young, I was passionate and excited and energized and wanted to help people in my community and change the world,” she told me in the 2018 interview. “What I’ve figured out is when you’re willing to work with people, even those with whom you sometimes disagree, when you work with people who are different from yourself, you can find common areas of agreement and achieve good things.”

Ms. Sinema has finally swung so far around that the people she used to disagree with are now her allies. Her old allies, who now disagree with her, no longer have any hope she’ll work with them.