Actually, it says that each state is guaranteed "...a republican form of government, ..." Article IV. section 4)

But "Beginning with the Constitution's adoption, America has been a Republic. But the dominant trend over the last two centuries has been to make it into a democracy as well, a representative democracy, also know as a democratic republic."

The current Republicans simply don't LIKE the word 'democratic' to be associated with anything positive, and this is an attempt to strip it from any labels and mention.