Hughes would probably not be a very progressive president--at least if one can judge from his 1916 campaign: the opposition to the Adamson Act, etc. William Allen White, who supported Hughes, nevertheless lamented, "He talked tariff like Mark Hanna. He talked of industrial affairs like McKinley, expressing a benevolent sympathy, but not a fundamental understanding. He gave the Progressives of the West the impression that he was one of those good men in politics—a kind of a business man's candidate, who would devote himself to the work of cleaning up the public service, naming good men for offices, but always hovering around the status quo like a sick kitten around a hot brick!"
https://books.google.com/books?id=cnU9AQAAMAAJ&pg=RA1-PA138
The Democrats in 1920 would probably run against Hughes from the left--accusing him of not cracking down hard enough on "profiteers", of acquiescing in an imperialist peace, etc. That doesn't mean that they would govern as radicals during the 1920's but they would still be to the left of the Republicans.
See
https://groups.google.com/d/msg/soc.history.what-if/kFsmJQuau10/7M1f-7wAmLoJ for an argument that while in a given election, the Democrats may nominate a relative conservative for president or the Republicans a relative progressive, the two parties are *structurally* different in a way that makes it inevitable that the Democrats as a whole are to the left of the Republicans as a whole.