I was struck by that comment by Arlen Specter. The fact is that moderate republicans have had opportunity to rebell time and time again and, typically, have not done so. I think it was this lack of nerve or confidence which has lead, since the time of Reagan, to the total purge of moderate Republicans outside of the state of Maine.
I said it started with Reagan, but Reagan was a big tent guy. He moved the party right, but brought along the moderates with him. Reagan's popularity lead to the huge growth of the Republican party in the 80s.
But the thing is, though Reagan was popular enough to win 49 states in 1984, The right wing of the Republican party was never so popular. A large segment of the population are not especially ideological, and will vote for candidates based on the "feel" they get from them, or based on current economic conditions. So the Republican party was on a roll and the moderates went along, not challenging the pro-life stance, the removal of the Equal Rights Amendment from the Republican Platform (it is hard to immagine but The Republicans supported ERA from 1944-1980.
Northeast Republicans never agreed with this rightwing social agenda, or with the radical anti environmentalism, or with the jingoistic foreign policy. Indeed many in the "Rockefeller" wing did not buy into the attempt right wing economics--Even the elder Bush called trickle down theory"economic voodoo". in 1976.
Actually now that I think of it, 1976 may have been the real starting point of the decline of the moderates. It was the one time there was an actual fight for the party, with Ford being the "moderate" choice. But even here, there was something odd. Prior to Reagan's challenge, no one would have called into question Gerald Ford's conservatism.
But the moderates stayed quiet after that. They did not challenge the dominant conservative ideology. Except for John Anderson in 1980 they did not put up their own candidates for president. They did not challenge the party platform. They let the national party define itself, even though there were huge numbers of Republicans who felt nothing more than aversion to the far right agenda (I speak mainly of the northeast, but I think this was also found in my state of Iowa and on the west coast). True, they may have lost the fight. Probably they WOULD have lost the fight. But by not doing anything, by just going with the flow, they guaranteed their loss.