We hear often from the hard right that Republican officeholders are squishy when it comes to principles compared to Democratic officeholders.
And based on non-partisan evidence...they're right.
Republicans are much more likely to work with Democrats than vice-versa. You can see that from the non-partisan work of the Lugar Center. A whopping 9 of 10 of the most bipartisan senators are Republican, while 8 of 10 of the least bipartisan ones are Democrat:
The simplest and most likely explanation is that the media media protects and allows extremism in Ds more than with Rs.
Regardless, it takes a lot more courage to be a conservative in Washington than a liberal given the twin pressures of elite opinion and the grass roots progressive groups (who camp out in offices, harass senators in elevators, etc...).