ALBANY, N.Y— A Democratic state senator who betrayed his colleagues by joining Republicans in a brief coup and was convicted of a misdemeanor assault charge for dragging his girlfriend around during an argument at his apartment says he'll go back to court to fight his expulsion from the Senate.

The New York Senate voted 53-8 on Tuesday night to oust Sen. Hiram Monserrate, a move the lawmaker called an injustice to the people who elected him.

Monserrate and another Democrat teamed with Republicans last summer in a coalition that resulted in a monthlong legislative gridlock before they switched back.

He then was convicted at trial last fall of dragging Karla Giraldo through his lobby in December 2008 but was acquitted of felony assault. A felony conviction would have automatically cost him his job.

Monserrate, of Queens, apologized for any discredit his conduct brought to the Senate, but he said Tuesday's vote was depriving voters of their right to choose a representative.

"We look forward to the court's intervening to protect the constitutional rights of the people that I represent," he said.

Attorney Steve Hyman said he and Monserrate will file a federal lawsuit challenging his removal as soon as possible. Hyman and Monserrate said they will seek court orders preventing officials from removing him or holding a special election to replace him.

Gov. David Paterson, also a Democrat, said he will call a special election for March 16 to fill the seat. Attorney General Andrew Cuomo's office is prepared to defend the Senate's decision, spokesman John Milgrim said.

Monserrate is separately appealing his criminal conviction.

Sen. Eric Schneiderman, a Democrat who chaired a special investigations committee that recommended censuring or expelling Monserrate, said Monserrate's conduct damaged the integrity of the Senate.