“This is to advise that Kathleen Vinehout just called to advise that she will announce tomorrow that she will not seek election as governor,” Matthews wrote to members Thursday just after 4 p.m.

Vinehout's departure leaves Madison School Board member Mary Burke as the only Democrat likely to mount a serious challenge to Gov. Scott Walker in November.

Vinehout, D-Alma, was actively exploring a possible run until severely breaking her arm in the Dec. 8 car crash. She underwent an eight-hour reconstructive surgery.

Vinehout was on her way to meet with prospective supporters when the crash happened.

Vinehout could not be reached for comment.

Matthews said in an interview that Vinehout told him in the past week that she was questioning whether she could take care of her own health and be a viable candidate.

“She told me a few days ago that things were iffy because she’s had 14 breaks in her arm,” Matthews said. “She’s got pins and screws and plates. She’s facing more surgery.”

Matthews, who runs one of the state’s largest teacher unions, said without Vinehout, there won’t be a Democratic primary.