Sen. Craig Thomas (R-Wyo.) remained in serious condition at National Naval Medical Center in Bethesda, Md., Monday evening as he underwent a second round of chemotherapy and fought infection related to his months-long struggle with leukemia.

Doctors didn’t provide a specific prognosis for Thomas, 74, but a statement from the Senator’s family indicated the medical team was having difficulty managing his blood cancer. Thomas was hospitalized on May 24 following routine tests to monitor his bone marrow and white and red blood cell counts.

“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 leukemia,” the Thomas family statement said.

Thomas’ wife, Susan, who was with him Monday along with children Patrick, Greg and Lexie, offered appreciation for the well wishes.

“At this difficult time, all we can do is give him as much love and support as possible,” Susan Thomas said. “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.”

Thomas was first diagnosed with acute myeloid leukemia in November 2006, just after sailing to a third Senate term with 70 percent of the vote. The popular Wyoming Republican began his treatment within days after learning of his condition and continued his therapies throughout the early part of the 110th Congress.

Doctors felt Thomas was having such a positive response to the treatment that they opted against administering a round of chemotherapy in March, Thomas’ office said. Meanwhile, Thomas was back to a regular vote and committee schedule in the Senate and continued to voice optimism about his recovery.

“When these things happen, you have to do what’s necessary to deal with it,” Thomas said in a January interview with Roll Call. “I must say I didn’t know what I was getting into. I was shocked and concerned because anything compared with that title is scary. I just look forward to getting back in the swing of things.”

Thomas said then that he was surprised when his cancer diagnosis came down since he had little forewarning and no one in his family has suffered from the malady. Initially, the doctors believed it was pneumonia.

Thomas isn’t the only Senator this Congress to undergo a major health crisis. Sen. Tim Johnson (D-S.D.) continues to recover from a brain hemorrhage he suffered last December. Johnson has yet to cast any votes this Congress, but he remains in rehabilitation and appears poised to seek re-election to a third term in 2008.

It is rare but not unprecedented for a sitting Senator to miss long periods of Senate business. Also, there have been instances when a Senator has been unable to fulfill the remainder of his or her term, the most notable recent example coming in December 2002, when then-Sen. Paul Wellstone (D-Minn.) died in a plane crash.

If it became necessary to replace Thomas, Wyoming law ensures that Senate Republicans will not lose a vote, as Gov. Dave Freudenthal (D) is required to select a replacement for the Senator from among three candidates submitted to him by the state GOP.

However, the individual chosen by the governor to fill the remainder of Thomas’ term would serve only until November 2008, giving the seriously threatened Senate Republican Conference another seat to protect in a cycle when it already is defending nearly twice as many seats as the Democrats.

Senate Democrats are defending only 12 seats this cycle, compared with a whopping 21 for the Republicans.

Should Thomas be unable to finish the remainder of his third term he won in 2006, Wyoming Republican Party Chairman Fred Parady confirmed that the 71-member state GOP central committee would select the names of three individuals from which Freudenthal would choose a successor.

But Parady said the state GOP has yet to work out the mechanics of how those individuals would be chosen, saying he hopes it doesn’t become necessary.

“We’ll deal with an opening when and if it occurs. We hope it doesn’t,” Parady said late Monday.

According to Wyoming law, the governor is mandated to inform the political party of the departed Senator immediately upon his receiving official notice of the vacancy. The state party in question then has 15 days to call a meeting of its central committee and settle on three potential replacements that meet the legal requirements to serve as a Senator.

Once the governor receives those three names, he has five days to choose one of them for the vacancy, and that choice would then serve until the next regularly scheduled general election.

Previously a state legislator, Thomas was first elected to the House in 1989, winning a special election to replace then-Rep. Dick Cheney (R-Wyo.), who resigned to become Defense secretary. Thomas successfully ran for the Senate in 1994 and won re-election to the seat in 2000 and again last year.

Thomas, an ally of Cheney with strong agricultural roots, has twice been mentioned as a possible Interior secretary, including last year when Gale Norton announced she was leaving the Bush administration post. Thomas later withdrew his name from consideration, and Dirk Kempthorne (R), the former Idaho governor, won the appointment.