Rep. Denny Rehberg’s fortunes plummeted by more than 80 percent in 2010, according to newly released financial disclosure forms, as the Montana lawmaker devalued ranch land damaged in a 2008 wildfire.

The Republican lawmaker — who has announced he is running for Senate — reported sharp reductions in the minimum value of his two largest assets, Rehberg Ranch and Rehberg Ranch Land and Livestock, which had been listed at a minimum worth of $5 million each in 2009.

In his newest report, Rehberg lists the minimum values of Rehberg Ranch at $1 million and Rehberg Ranch Land and Livestock at $500,000.

Those reductions drop his minimum net worth from $10.9 million in 2009 to $1.8 million in 2010, a reduction of more than 80 percent. Rehberg had been among the 50 richest Members of Congress last year, but he will not make that list this year.

Rehberg’s office could not be reached immediately Thursday, but the Montana lawmaker indicated in his 2009 financial disclosure that his Rehberg Ranch investment would be reappraised “due to fire damage and real estate market downturn.”

According to an Associated Press report earlier this month, Rehberg filed a lawsuit against Billings, Mont., over its handling of a wildfire on his property in 2008.

Rehberg told the AP that the fire damaged 1,200 acres of land at Rehberg Ranch Estates, which the lawmaker and his spouse planned to develop.

“It was never intended to be anything more than trying to protect her business, our business,” Rehberg told the AP. “We are not blaming the firefighters in any way, shape or form.”

The Montana lawmaker is seeking the GOP nomination to challenge first-term Sen. Jon Tester (D) in 2012.