Washington, D.C. ? Sen. Pete Domenici of New Mexico, an influential Republican voice on budget issues for a generation, intends to retire at the end of his term next year for health reasons, party officials said Wednesday.

The 75-year-old, six-term lawmaker plans a formal announcement today in his home state.

Domenici began informing associates late Wednesday he has frontotemporal lobar degeneration, a progressive disease that in some forms can cause dysfunction in the parts of the brain important for organization, decision-making and control of mood and behavior.

Domenici would be the fifth Republican senator to decline to seek a new term, giving Democrats an opportunity to expand their narrow majority in the 2008 elections. GOP Sens. John Warner of Virginia, Chuck Hagel of Nebraska, Wayne Allard of Colorado and Larry Craig of Idaho have previously announced plans not to run again.