According to the polls that DougR cites, there is only majority support for Bush's plans IF it is done multi-laterally and with UN support. When you take away the latter, the poll numbers drop precipitously.

Just to throw a wrench in the works, I see the BBC is reporting today that:

"The UK would not join American unilateral military action against Iraq, according to International Development Secretary Clare Short.
Ms Short said the UK had a duty to try to keep the world united over the Iraq crisis and ensure the danger of Saddam Hussein was only tackled through the United Nations.

Other ministers have shied away from saying whether or not the UK would join America if it decided to act alone against Iraq."

I believe Bush could still get away with not having UN support, if he can keep Blair with him.   But if both the UN and the UK refuse to support an invasion, Bush and the boys will be WAY out on a limb. I'm not convinced they would do it without clear, enthusiastic support from one or the other. I think what they want very badly, for the sake of the upcoming 2004 elections, is the support of both the UN and the UK.