MADISON, Wisconsin (Reuters) - Wisconsin state Senate Democrats boycotting a vote on a plan to curb the union rights of public employees left the state on Thursday in an attempt to force majority Republicans to negotiate a compromise, an online political news service said.

Democrat Sen. Jon Erpenbach told WisPolitics that he believed all 14 Senate Democrats were out of the state by early Thursday afternoon. There are 19 Senate Republicans, and a quorum of 20 is needed to vote on the issue.

Erpenbach would not disclose where he was or how many of the Democratic senators were with him.

"We were left with no choice," Erpenbach said. Erpenbach would not say how long they were prepared to remain outside Wisconsin.

"The question is when are the Republicans going to sit down seriously with the other side on this issue and try to work something out," Erpenbach told WisPolitics. "When are we going to be reasonable about this and slow things down?"

"This isn't anything that we do lightly at all," Erpenbach said. "This isn't a prank. This isn't a joke. This is Democrats standing together saying slow down."

Republican Gov. Scott Walker has proposed sharply curbing the bargaining rights of public unions in order to make budget savings. The proposal was to be debated and possibly come to a vote in the state Senate on Thursday. Republicans hold a majority of both the Senate and the state House of Representatives.

(Reporting by Jeff Mayers, Writing by Mary Wisniewski, Editing by Greg McCune)

Mochila insert follows.