If moderate is another way to say “no-principle, deal-making opportunist,” Baker was a leading moderate from 1966 on. In 1982, he labeled himself a “moderate to moderate conservative,” whatever that means.
To be fair to Baker, I don’t suppose he ever thought he was retarding the Republican agenda with his political shape-shifting. When he first got to the Senate, liberal Republicans like Edward Brooke (R-Mass.), still walked the chamber, as did conservative Democratic senators. Dealing across party lines didn’t automatically mark you as a traitor to your cause. Indeed, a good bargain struck with a member of the opposing party could prove you a loyalist to your cause.
This “bipartisan comity” and “talent for compromise,” which the Post obituary identifies with Baker, wasn’t just a function of his warm personality. Everybody was dealing back in the 1960s and the 1970s. Republicans like Baker, being in the legislative minority, had no choice but to compromise with the Democrats if they hoped to reap patronage for their constituents.