Most Americans are saying the invasion of Iraq was wrong. Are they serious? Why then did they reelect George Bush?

Similarly in the UK, you could ask why did they re-elect Blair?

The answer is that the Iraq war is just one issue on which people vote - others (at least in the UK) include (but are not restricted to) the economy, health services, law & order, education, transport, defence and foreign policy (which I guess includes the Iraq war).

The voters presumably weigh up the various things that are important to them, and vote for who they believe will best represent their interests. I guess although many people disapprove of the Iraq war, they don't feel it outweighs domestic issues that probably affect them more directly.

Whether they got their assessment right or not is a different question.