Republican Sen. Craig Thomas is in the hospital in serious condition as he battles a leukemia with a second round of chemotherapy.

Thomas entered the National Naval Medical Center in Bethesda, Md., before the Memorial Day recess.

"Doctors have been administering a second round of chemotherapy to control the disease, but the senator's blood cancer has proven resistant to their most recent efforts and he continues to struggle with infection in addition to the leukemia," said Susan Thomas, wife of the senator, in a statement released Monday.

"At this difficult time, all we can do is give him as much love and support as possible. The support and prayers of Wyoming folks have made a tremendous difference to us. It has meant everything to Craig, and I know it helps him today," she continued.

Along with Susan at the hospital are Thomas' sons Patrick and Greg and daughter Lexie.

Thomas, 74, spent a month in the National Naval Medical Center last year after his initial diagnosis but returned to work in December. At the time, Thomas, who is suffering from Acute Myeloid Leukemia (AML), the most common type of leukemia, which is a cancer of the blood and bone marrow, told FOX News he was feeling good, especially after just winning his third Senate term.

Staffers on Capitol Hill refer to Thomas as the "nicest man in the Senate," and he is known for his kind words and good sense of humor. He frequently jokes with the elevator staff who hold the doors for senators going to and from votes, the attendants who guard the doors to the Senate floor and others.

Thomas was elected to the Senate in 1994 after serving five years in the U.S. House. He was elected to the House seat in a special election to replace Dick Cheney, who at the time became President Gerald Ford's defense secretary.

Reports are conflicting about the seriousness of Thomas' condition. Sources told FOX News that the senator is in pretty bad condition but that details of his health are being kept very close to the vest by the family. Thomas spokesman Cameron Hardy said the office is awaiting an update on the senator's condition, and if warranted, would put out a statement.

A spokesman for the Wyoming secretary of state told FOX News that in the event of the senator's passing, the governor chooses from a limited list of qualified candidates submitted from the replacement senator's own party. The governor of Wyoming, Dave Freudenthal, is a Democrat but state law removes any possibility of a party switch in the replacement process.

FOX News' Trish Turner contributed to this report.