As long as Democrats controlled Congress or the White House, Republicans could tell conservatives they deserved support because of what they would do, someday. Now we know what they do when they have control. Their agenda comes from Big Business, not from grass-roots conservatives.
There's the heart of the matter. The Republican Party (and the Democratic Party for that matter) consists of several factions with their own interests. You can hold such a thing together over the long haul only if the factions see their interests advancing. In the Republican Party, the Pro-Lifers aren't happy with stealth judges and Arlen Spector's chairing of the judiciary committee, the Small Government types aren't pleased with the growth of government, and the law and order Republicans aren't thrilled with the Border and the Botched Response to Katrina (not that they let the locals off the hook).