Sen. Bill Cassidy of Louisiana walked down a hallway flanked by three members of the U.S. Capitol Police, each ready to ward off protesters who might confront him about his health-care bill.

It was an unusual day for the little-known Republican senator, spent shuttling from interview to interview, then to the Senate Finance Committee, where he would pitch his approach to dismantling Obamacare to his colleagues, and finally to a CNN stage where he subjected himself to a nationally televised, 90-minute debate despite his bill's collapse hours earlier.

Was he still hoping to bring the measure back from almost-certain defeat?

Hard to say.

The Cassidy-Graham proposal is unpopular with voters. It is opposed by the medical establishment to which Cassidy, a physician-turned-politician, once belonged. Experts estimated that it would kick tens of millions off their health-care plans. Even some members of Cassidy's own party are skeptical.

At what point does a quixotic effort become masochistic? And what would make Cassidy, or anyone for that matter, want to remain its public face?

One clue emerged in an interview with The Washington Post on Monday: Cassidy is determined. Relentlessly, strangely determined.

"Keep plugging, keep plugging and never quit," he said. "Two years ago, people thought I was Don Quixote. A month ago, people thought things were dead. Two weeks ago, people smiled. And now folks say, 'Wow, they may still pull it off.' If you keep your head down and keep plugging, good things happen, and that's my goal."

In the committee, Cassidy had hoped to revive a bill that had hovered near collapse for four days. He had revised the text to appeal to holdout senators. He would try to win approval from the heavyweights on the panel, where he is the most-junior member. He didn't have much to work with.

"I was sent here to work," he said at one point. "If this [the bill] is the only means by which I can do so, I shall."

At another moment: "For three years, I went around to Democratic colleagues and met with you and asked, 'Can we please work together?' For three years, I was basically told, 'Nice try.' "

[Senate GOP effort to unwind the ACA collapses Monday]

As the hearing began, the bill was holding on to a slim chance of success. Cassidy would find out before the committee adjourned, five hours later, that this chance was all but gone.

The first-term senator cuts an unusual figure in the upper chamber. He stands out at over 6 feet tall, with strong features and a shock of gray hair. Described as friendly but without close friends, he ranges around the Capitol with his headphones in, avoiding small talk. Before the health-care debate, he was not known for a signature issue — nor for a calling card among his colleagues.

That all changed in May after Cassidy's encounter with Jimmy Kimmel.

Cassidy had not even been part of Senate Majority Leader Mitch McConnell's official working group on health care early this year, despite his stated interest in the issue and his medical degree. He partnered with Sen. Susan Collins (R-Maine) on an Affordable Care Act replacement bill, but the legislation had aroused little interest.

He finally managed to join the debate after coming up with a standard for judging Republican legislation to replace the ACA. He would call it the "Jimmy Kimmel test" — an outgrowth of the late-night host's plea to Congress to protect the ACA, which began after his son was born with a heart defect. Cassidy declared, after Kimmel's advocacy made headlines, that he would not support a bill that failed to guarantee that patients like Kimmel's baby would receive health coverage regardless of income.

Cassidy initially won praise from liberal advocates. But by mid-September, he had opted to become a lead sponsor of a new proposal offering dramatic cuts to Medicaid and leaving many coverage decisions to states.

Cassidy claimed that his position had not changed since May.

"My patients had terrible diseases and multiple chronic conditions, and my life's work was how to care for these fellow Americans," he said in a video posted on Twitter over the weekend, referring to his work with Louisiana's uninsured patients. "[Cassidy-Graham] accomplishes this by other means."

Kimmel — and many others — concluded otherwise.

"This guy, Bill Cassidy, just lied right to my face," Kimmel said last week on his program, "Jimmy Kimmel Live!"

[@PKCapitol: The latest repeal effort was bad news from the start]

Cassidy claimed after Kimmel's riff that the comedian didn't understand his proposal. But the legislation is arguably more extreme than the bills Republicans considered in the summer. It goes further in its cuts to federal health-care spending, aims those cuts more directly at states that expanded Medicaid under the ACA and allows states to use waivers to opt out of the law's consumer protections.

Mistaking disagreement for misunderstanding is typical for the awkward, slightly nerdish Cassidy, 59, who invokes his background in medicine at just about every opportunity.

A liver specialist by training, Cassidy helped create a free clinic for Baton Rouge's working poor and turned an abandoned Kmart into a makeshift medical facility after Hurricane Katrina.

Some in Louisiana Republican politics have speculated that it would be Cassidy's wife, Laura, a retired breast-cancer surgeon, who ultimately made the run for office. The two met during their medical residencies in the 1980s. He calls her his closest adviser.

"On health care, she has very much lived what I've lived," Cassidy said.

"She was in a publicly run system and noticed the resources always lagged behind those of a private hospital. . . . She understands the needs for coverage as do I, having patients who for whatever reason were uninsured and their breast cancer was delayed, both diagnosis and treatment, sometimes with terrible results," he said. "So she shares my passion."

When Cassidy sat down to testify at Monday's 2 p.m. hearing, the math for his bill was already difficult. With 52 seats and the tiebreaking vote of Vice President Pence, Republicans could lose only two GOP votes to pass most legislation. Sens. John McCain (R-Ariz.) and Rand Paul (R-Ky.) had already come out against the bill. That left Sens. Susan Collins (R-Maine) and Lisa Murkowski (R-Alaska), whom Cassidy had tried to lure with extra funds for their states in a new version of the bill released Monday.

Dressed in a dark suit, sky-blue shirt and navy-blue-and-pink striped tie, Cassidy entered the hearing room as protesters in the hallway chanted, "No cuts to Medicaid!"

The next five hours passed slowly, as Cassidy entered the weeds of health-care policy to defend his bill, clause by clause. At one point, Sen. Charles E. Grassley (R-Iowa) referred to him as "Dr. Kennedy," mistaking Cassidy for Louisiana's junior senator.

After one particularly heated exchange, Finance Committee Chairman Orrin G. Hatch (R-Utah) called for the panel's Democrats to ease up on Cassidy.

"He's a doctor, and he understands this probably better than anybody in this room," Hatch said. "Let's show some respect for Senator Cassidy. This is not easy for him."

Hours later, news broke that Collins was a firm no on the bill. Cassidy continued to implore Democrats to support the legislation, bringing out color-coded maps to make his points.

Hatch said he would suspend the hearing after another round of questions.

"Let's face it," he said. "We're not getting anywhere."

Paige Winfield Cunningham, Sean Sullivan and Natalie Jennings contributed to this report.

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