The mythical refuge GOP moderates do not exist in any real significant numbers, but even if that wasn’t the case, they aren’t the factor driving the Democrats to the right. That’s a product of 50 years of neoliberalism and the DNC being owned by their donor class.
Once you understand that, you can fit all of the other contributing factors under those two umbrellas, such as how Democratic administrations have refused to provide any real legislative solutions to the judicial branch’s removal of union’s ability to accumulate and retain real political power.