Why didn't Republicans become the progressive/ liberal party?

[°Leprechaun]

The Democratic party wasn't always as liberal as it is now, but that doesn't mean that the GOP had to be liberal.

The Democrats were once the establishment party and the Whigs anti establishment; many Whigs became Republicans.

Being anti establishment isn't the same as being liberal.

I do think Bryan was liberal for his time although obviously very conservative religiously.

[°Leprechaun]

The Democratic party wasn't always as liberal as it is now, but that doesn't mean that the GOP had to be liberal.

The Democrats were once the establishment party and the Whigs anti establishment; many Whigs became Republicans.

Being anti establishment isn't the same as being liberal.

I do think Bryan was liberal for his time although obviously very conservative religiously.

Curious what you mean about the Whigs?  Most contemporary attack ads against them from Democrats seem to have painted them as a party of monied interests (in bed with Wall Street and the railroad companies), anti-immigration and morally judgmental.

I think that Whigs were formed as an opposition party to Jackson and that Jackson was seen as an establishment President.

[°Leprechaun]

Religion is probably one of the big factors.

It is also useful to look at the maps of POTUS elections.

The southeastern, northeastern and western states are a good example.

Start with Vermont once the most Whigs/GOP state.

Today only DC is more Democratic than Vermont.
The best example is 1976. The west was very GOP.
The southern states for the most part went for Carter.
A lot changed from 1876-1976.
Obviously the GOP has moved to the right over the last 100 years.

Although FDR was more liberal than the GOP today, but I think Dewey was liberal for his time. There are probably few Rockefeller (who was VP) Republicans today.