Two days after the election, Senator Thomas said he had been diagnosed with leukemia.
Governor Dave Freudenthal, a Democrat, will appoint a successor from one of three finalists chosen by the state Republican Party.
"Wyoming had no greater advocate, taxpayers had no greater watchdog, and rural America had no greater defender than Craig Thomas," Senate Republican leader Mitch McConnell said last night. "The Senate is a lesser place without Craig here, but the state of Wyoming and our nation are much better places because he was here."
Senator Thomas was a low-key lawmaker who reliably represented the interests of his conservative state, often becoming involved in public lands issues. He worked in behind-the-scenes posts to oversee national parks, including Yellowstone in Wyoming.
He was also an advocate for domestic energy and minerals production. He worked to protect Wyoming's mining industry from foreign competition and backed efforts to get a federally funded coal gasification plant built in the state.
Wyoming's other senator, Republican Mike Enzi, said he was "so stunned" by the death.
"Craig was the core of our delegation," Enzi said. "He was a fierce advocate for Wyoming."
After his first round of chemotherapy, Senator Thomas returned to the Senate in December, a month earlier than expected. A few months later, he said he felt better than he had in a long time. But he returned to the hospital for a second round of chemotherapy in May.
Senator Thomas entered Congress in a special election in 1989 to replace Dick Cheney in the House of Representatives when the current vice president was named defense secretary by the first President Bush.
In 1994, Senator Thomas won his first Senate race by beating former Governor Mike Sullivan 59 percent to 39 percent. Senator Thomas was reelected by a wide margin in 2000, winning 74 percent of the vote.
Senator Thomas had previously served five years in the Wyoming Legislature.
He was born in Cody, Wyo., and grew up on a ranch. He graduated from the University of Wyoming with a degree in agriculture, then served four years in the Marines.
He also was vice president of the Wyoming Farm Bureau and general manager of the Wyoming Rural Electric Association.
Senator Thomas leaves his wife, Susan, and four children.
