The “teabaggers” (

) didn’t drive centre conservatives left, but they did drive elements of the Democratic Party left, and in part caused this reaction that allowed the progressive/socialist wing of the party to slip into the mainstream.
Conversely, the democrats shifting left into inane identity politics caused a hard right shift in reaction from the Republican Party and brought alt-right populism into the mainstream.
Basically the two parties and their supporters being so reactionary in the 21st century has made the partisan divide untenable. A truly centre position for an American politician today is anathema.