Oh, it would be a disaster all right, both domestically and internationally. But I don't think it would lead to the downfall of the Constitutional order. There would still be an election in 2008, and the Democrats would likely win it and win it big.
The Constitiution was born out of outrage. Perhaps 4 more years of bushco would generate outrage on an appropriate scale, and produce tectonic changes in our system rather than this incremental pushing and shoviing around the center. I mean we already suffered with Reagan and Nixon; clearly we don't learn things easily, or remember well--which in some circles defines stupid.