JANESVILLE, Wis. -- A possible speedup in the shutdown plans for General Motors' Janesville plant prompted Sen. Herb Kohl to issue a statement calling it a "dark day" for the community and the U.S. economy.

But there's no confirmation from GM on when the Janesville plant will halt production of the sport utility vehicles made there. Gov. Jim Doyle and Rep. Paul Ryan said GM hasn't ruled out a proposal to make other vehicles in Janesville.

“I will continue to fight for the General Motors' workers and provide the resources necessary to help the Janesville community as the Department of Workforce Development and the Department of Commerce continue their work to provide services and assistance to the workers and their families," said Doyle in a news release.

GM announced in June it would idle the Janesville plant and three others by 2010. Then, last week, GM said one of the four, in Moraine, Ohio, would close Dec. 23.

The company may announce as early as next week that more production cuts and plant closings are on the horizon.

GM's shares have lost nearly half of their value this week, plunging to their lowest level in more than 58 years.