Further explanation which I think is insightful: Washington DC is culturally liberal. The institutions that long-tenure senators interact with daily: the press corps, the think tanks both parties actually attend (Brookings, CFR, Aspen, Carnegie), the schools their kids go to, the dinner parties are all left-of-center. That creates asymmetric social pressure: A Republican who drifts left gets rewarded. The press calls them a "statesman," a "maverick," a "reasonable voice." They get invited to the bipartisan events. Their kids don't get weird looks at Sidwell Friends. McCain got this. Collins gets this. Murkowski gets this. There's a whole media infrastructure that celebrates the "heroic moderate Republican." A Democrat who drifts right gets... nothing. There's no equivalent conservative institutional establishment in DC that would celebrate them. No black-tie dinner full of journalists who would applaud them for being "courageous" by moving right. Their base would primary them. The press wouldn't reward them. So they don't. The incentive structure is one-directional. DC's Overton window is set by the press corps and the permanent institutions, and it pulls left. Republicans face constant pressure to accept that frame as the price of being taken seriously. Democrats are already inside the frame.

Whew. Finally committed the code. This was the biggest project I've done, easily. It will launch along with the soon-to-come DataRepublican 2.0 which is more mobile forward - working with @watilo to target a launch tomorrow.