Story Highlights • Sen. Craig Thomas, R-Wyoming, died Monday, says family spokesman

• He had been battling leukemia since November

• Thomas first came to Congress in 1989

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(CNN) -- Republican Sen. Craig Thomas of Wyoming, who had been battling leukemia since November, died Monday night at the National Naval Medical Center in Bethesda, Maryland, a family spokesperson told CNN. He was 74.

Reacting to the news of Thomas' death, Senate Republican Leader Mitch McConnell expressed his condolences, saying "the state of Wyoming and our nation are much better places because he was here."

"Wyoming had no greater advocate, taxpayers had no greater watchdog and rural America had no greater defender than Craig Thomas," McConnell said.

Thomas was elected to a third term last year. The vacancy caused by his death will be chosen by Wyoming Gov. Dave Freudenthal, a Democrat. However, the seat will not switch parties because, under state law, Freudenthal is required to pick a new senator from a list of three candidates submitted by the GOP state central committee.

When he was re-elected in November with 70 percent of the vote, Thomas was in the hospital undergoing chemotherapy for his leukemia. After three weeks of treatment, he returned to work in the Senate in early December.

But the leukemia proved resistant to the treatment, forcing doctors to administer a second round of chemotherapy. Earlier Monday, Thomas' office had reported he was in serious condition, both from the leukemia and an infection.

Thomas first came to Congress in 1989, when he won the Wyoming's lone House seat in a special election after now-Vice President Dick Cheney resigned to become defense secretary. In 1994, he won an open Senate seat and easily won re-election in 2000 and 2006.