Domenici is the fourth Senate Republican to announce his retirement.
WASHINGTON (CNN) — Veteran Republican Sen. Pete Domenici of New Mexico will announce Thursday that he will not seek re-election to a seventh term next year, opening up yet another competitive seat for the GOP to defend, three Republican sources told CNN Wednesday.
Domenici, 75, who was first elected to the Senate in 1972, will make his retirement announcement at 4 p.m. (6 p.m. ET) at St. Mary's School in Albuquerque, the grammar school he attended as a boy.
A Domenici aide told CNN that the senator will be very specific about his reasons for retirement, which the aide said have nothing to do with either his poll numbers or the scrutiny Domenici has faced over the firing of a federal prosecutor in New Mexico.
The departure of Domenici is a further blow to Republican chances of retaking the Senate in 2008, opening up fifth GOP seat to defend in a state where Democrats are highly competitive.
Sitting Republican senators in Virginia, Colorado and Nebraska have also announced their retirements, and Sen. Larry Craig of Idaho, caught up in a sex scandal, has indicated he won't seek re-election and may leave the Senate sooner if his battle to overturn a guilty plea to disorderly conduct charges isn't successful.
In order to regain control of the Senate, Republicans must make a net gain of just two seats. However, they are defending 22 seats - including the five open seats - while Democrats have just 12 to defend. While Democrats haven't won a Senate seat in Idaho in more than 30 years, the open races in New Mexico, Virginia, Colorado and Nebraska are likely to be competitive.
Domenici, the longest serving senator in New Mexico's 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 eight U.S. attorneys, including David Iglesias of the District of New Mexico.
Iglesias has said Domenici called him prior to 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 believes he was dismissed a month later for refusing to expedite the case.
Domenici has admitted calling Iglesias, but he has denied trying to influence the investigation.
In his years in the Senate, Domenici has served as the chairman of Budget Committee, as well as the Energy Committee, where he is now the panel's ranking Republican.
In July, he broke with the Bush administration on 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.
–CNN Congressional Correspondent Dana Bash