Chuck Hagel is one of two Republican co-sponsors of a Democratic Senate resolution opposing extra US forces. Photograph: Stefan Zaklin/EPA

“It is wrong to put American troops in the middle of a civil war – Iraqis killing Iraqis, Shias killing Sunnis, Sunnis killing each other, Shias killing each other,” Mr Hagel, a Vietnam veteran, told ABC television. “To feed more American troops into this bloodbath is wrong.”

Mr Hagel is viewed as a potential contender in the 2008 presidential elections, which would make him very attentive to opinion polls showing a majority of Americans opposed to the troop increase.

Such calculations could also underlie the decision by a conservative Republican senator, Sam Brownback, to break with the White House. He is due to announce his candidacy in 2008 from his home state of Kansas tomorrow. On his return from a trip to Iraq on Wednesday, he called for a new round of diplomacy instead of more troops, arguing that the Iraqi government was not sufficiently engaged with trying to bring peace.

“All of this suggests that, at the present time, the United States cares more about a peaceful Iraq than the Iraqis do. If that is the case, it is difficult to understand why more US troops would make a difference,” Mr Brownback told the Senate.

With many Republicans worried that Iraq could prove as devastating in the 2008 presidential vote as it did in 2006, the party has been hard-pressed to maintain loyalty with the White House. A number of party rebels are from states that voted for Democrats in November, and are afraid of losing their seats in 2008.

The White House spokesman, Tony Snow has indicated that those who embarrass the administration will not face an easy time. “What message does Congress intend to give?” he asked. “And who does it think the audience is? Is the audience merely the president? Is it the voting American public or, in an age of instant communication, is it also al-Qaida?”