I would say the all time Republican low point for the GOP was 1932-1938 and second was 1964-1966
Depends on how you look at it, I suppose. In the early/mid 1960s, it was a low point for the GOP in the Senate for sure, but the House and states, it wasn't as bad as the 1970s, and early-mid 80s with the states:
https://en.wikipedia.org/wiki/Party_divisions_of_United_States_Congresseshttps://en.wikipedia.org/wiki/Political_party_strength_in_U.S._statesDemocrats held large majorities in Congress for longer during the Nixon backlash right up to 1980. And the GOP's position in state legislatures was absolutely terrible from 1974 -> 1992. They got hit with two huge waves back to back because of Nixon. It takes time to undo that damage.
It's just a bit trickier because Reagan was undeniably a sign of the country moving towards Republican rule, but that would come later. While he was president, he was basically like a boot on the neck of state-level Republicans, who like Democrats under Obama would have a hard time clawing back power until Republicans stopped occupying the White House.