At the Capitol on Tuesday, he spent much of the day declaring that he was going to fight to keep his seat, and that the very rule of law was at stake.

“There was one, sole conviction on reckless assault misdemeanor, which is under appeal,” Mr. Monserrate said. “This is not about the underlying incident.”

What, then, was shaping the debate about throwing him out?

He spoke in a low voice: “There’s a lot of politics driving this.”

The point was inarguable. Mr. Monserrate had committed multiple political felonies, and Tuesday was as much the day of reckoning for them as it was for the crime he was convicted of.

Image Credit... Nathaniel Brooks for The New York Times

During his 13 months as a state senator, both parties found Mr. Monserrate to be a useful bug to have in the bed. He was elected as a Democrat in 2008, when the party gained a majority in the Senate for the first time in decades. Six months after taking office, he switched sides with Senator Pedro Espada Jr., who was given various emoluments and a splendid title by the Republicans so that they could recapture control of the Senate.