MADISON, Wis. (AP) -- State election officials say challenges have been filed to four of the six candidates running in elections to recall three Democratic state senators.

The GAB said Friday some of the challenges question the validity of signatures on the candidates' nomination papers.

The four candidates have until Monday at 8 a.m. to respond to the challenges.

The recalls stem from Republican Gov. Scott Walker's efforts to remove collective-bargaining rights from most public workers. Six GOP state senators are being targeted for supporting the measure, while the three Democrats are being targeted for fleeing the state to stall it.

Both candidates challenging Democratic Sen. Robert Wirch have been approved. The four challengers to Sen. Dave Hansen and Jim Holperin have yet to be finalized.