WASHINGTON (CNN) -- Republican Sen. Pete Domenici will not seek re-election next year because he has progressive, incurable brain condition that might prevent him from completing another six-year term, he announced Thursday.
Republican Pete Domenici is the longest-serving U.S. senator in New Mexico's history.
"I had to consider whether I could, in good conscience, run for re-election and serve you as well as you deserve," said Domenici, 75, who made the announcement in the gym of an Albuquerque, New Mexico, grammar school he attended as a boy.
"I concluded it would be wrong to ask New Mexicans to support me if I could not pledge that I could ably serve another full term."
Domenici was first elected to the Senate in 1972 and was re-elected five times. He said that for two years he has had frontotemporal lobar degeneration, or FTLD, a deterioration of brain tissue that can lead to personality changes, difficulty with speech and dementia.
While the condition has had "very little impact" on him, Domenici said a September checkup showed a slight progression of the condition. He said the erratic and unpredictable nature of the illness prompted his decision to retire.
However, the senator said he is still well enough to do his job and has "no doubts" he can finish his term, which ends in January 2009.
"We're going to do our very best with those 15 months to do something spectacular," he said.
Domenici concluded his remarks with a call for more research into brain diseases.
"No cure for my disease exists yet, but if we work hard enough, we may be able to find a way to cure people with diseases of the brain," he said.
FTLD is an unpredictable brain dysfunction, the symptoms of which can vary. In some cases, it can cause problems with organization, decision-making, mood and behavior, but it also is known to have effects on language, comprehension and motor skills.
Four prominent neurologists who are experts on the condition told CNN that in their view, it will be difficult for someone with FTLD to continue working.
"They would be prone to have poor judgment and make mistakes," said Dr. David Knopman of the Mayo Clinic, who said he encourages FTLD patients to leave their jobs.
Domenici's departure opens up yet another Senate seat that Republicans must defend next year, in a state in which Democrats are highly competitive.
However, one potentially formidable Democratic candidate, Gov. Bill Richardson, will not run for Domenici's Senate seat and will continue his quest for his party's presidential nomination, a senior political adviser to the governor said Thursday.
Sitting Republican senators in Virginia, Colorado and Nebraska also have announced their retirements, and Sen. Larry Craig of Idaho, caught up in a sex scandal, has said he won't seek re-election. He may leave the Senate sooner. A judge Thursday denied his request to withdraw his guilty plea to a disorderly conduct charge stemming from his arrest in an airport men's room sex sting.
To regain control of the Senate, Republicans must make a net gain of two seats. However, they are defending 22 seats -- including the five open seats -- while Democrats have 12 to defend.
Domenici, the longest-serving senator in New Mexico history, hasn't faced a competitive challenge to his seat in 30 years. However, in recent months, he has come under scrutiny for his role in the controversy over the firings of U.S. attorneys, including David Iglesias of the District of New Mexico.
Iglesias has said Domenici called him before the 2006 midterm elections to ask about the progress of a corruption investigation involving Democrats.
The prosecutor has said he felt the senator was trying to pressure him to bring down indictments before the election, and Iglesias said he thinks he was dismissed a month later for refusing to expedite the case.
Domenici has admitted calling Iglesias but has denied trying to influence the investigation.
In his years in the Senate, Domenici has been chairman of the Budget and Energy committees. He is the ranking Republican on the latter panel. He also has taken the lead on mental health issues, helping Sens. Ted Kennedy and the late Paul Wellstone pass a mental health parity act.
In July, Domenici also became one of a handful of GOP senators to break with the Bush administration over the Iraq war, saying he believed the United States should start moving toward a withdrawal. However, he has continued to oppose Democratic efforts to force a troop departure. E-mail to a friend
CNN's Elizabeth Cohen and Mark Preston contributed to this report.