The decision reflects growing public dissatisfaction with the war and the Republicans’ heightened concerns that it could hurt their chances in November’s midterm elections.

The move to allow an open discussion on Iraq – which consistently ranks first when Americans are asked what is the most important issue facing the country – comes amid renewed soul-searching over the war. This has been prompted in part by reports that marines may have killed unarmed civilians last year in Haditha, west of Baghdad.

While House leaders are ready to air the issue, the rest of the party’s leadership appears to be focused on other matters.

President George W. Bush has centred his remarks in recent days on gay marriage and immigration reform.

The Senate blocked on Wednesday a proposed constitutional amendment to prohibit gay marriage, and is expected to turn its attention to repeal of the estate tax, two issues that were scheduled for debate to boost Republicans’ standing among conservatives.

But the announcement by Dennis Hastert, speaker of the House, about the Iraq debate shows Republican leaders are wrestling with how to confront the issue. “We’ll spend the day, maybe more than a day, talking about the principles of what we’ve done and why we’ve gone into Iraq, and the goals and views of fighting against terrorism,” Mr Hastert said.

The text of the resolution to be debated has not yet been decided, nor have Republican leaders decided whether they will permit Democrats to offer any amendments. At the same time, some congressional Republicans are stepping up their oversight of the executive branch. John Warner, chairman of the Senate armed services committee, said this week that his panel would soon hold hearings into the allegations of a massacre, and subsequent cover-up, in Haditha.

Arlen Specter, chairman of the Senate judiciary committee, sent Dick Cheney, the US vice-president, an angry letter on Wednesday, complaining that he had interfered with the committee’s examination of Mr Bush’s controversial electronic surveillance programme.