We're going to catch some flak for it, but I agree with you, although he didn't destroy the party - he just weakened it greatly. His failure to understand that the "New Tone" was EXACTLY the wrong approach to take with Democrats in Washington even after eight years of their sabotage of everything he tried to do was his primary mistake, IMHO.
I'd say that was a mistake, but the primary one was his utter failure to defend himself from the lies that were told about him. He seems to believe he is above having to answer. By not refuting the lies, the lies have become "common knowledge".
Lies like, we went into Iraq on false pretenses. The war was all about the presence of WMD. The war has been a disaster. He has mismanaged the economy. Etc, etc.
Especially in the presence of a media that constantly lies about him, a President MUST communicate. Bush has failed to do so and has allowed the current anit-Bush, anti-Republican mood to exist.