By Rob Watson BBC Washington correspondent

There is a clear sense in this most political of cities that this could get very nasty for the Bush administration.

Just how nasty probably depends on the extent of White House involvement in the leaking of the identity of Joe Wilson's wife.

Accusations could be disastrous for Bush

If it could be shown that either President Bush or any of his closest aides were involved in the leaks, it could be disastrous for the administration.

So far though, White House officials are insisting neither the president nor his closest political adviser Karl Rove had anything to do with it.

Indeed, a presidential spokesman has said there is no evidence of any White House involvement beyond the reports in the media.

But both the Washington media and the president's political opponents are sensing blood.

The White House is under siege from reporters asking awkward questions while Democrats are demanding congressional inquiries and even the appointment of the same kind of independent counsel used to investigate presidents Clinton and Nixon.

Overall, it could not have come at a worse time for President Bush.

Opinion polls show his popularity at its lowest since the 11 September attacks.

Much of the drop in support, according to the polls, attributable to his post-war handling of Iraq.