The kind of hidden thing here is that, in few party systems like the US, the parties are composed of lots of different beliefs, the severity of which are highly

The kind of hidden thing here is that, in few party systems like the US, the parties are composed of lots of different beliefs, the severity of which are highly variable. For example, if I strongly believed abortion is murder, I would hold my nose and vote Republican every time, but that doesn't mean the fiscal responsibility wing of the party cares about that issue in any meaningful way, they just vote for it because in return I vote for their tax cuts even though I'd really prefer 2000 dollar checks in perpetuity. The reason people stick around is there's meaningful balance, however if the balance goes out of whack and the party became solely about anti-abortion at the expense of the fiscal responsibility, why would a fiscal responsibility person stick around? This is what happened to British Labour in the late 70s where the party started becoming solely about radical socialism (which was anti-Europe), and a number of high profile pro-Europe members left to form their own party, or more recently a number of pro-Europe Tories defecting to join the successor to said party. In defense of OP though, there are plenty of Labour politicians who were radical socialists in 1979 who became bonafide Blairites by 1997, there are absolutely politicians who just go where the political wind blows (in the case of someone like Scarborough that could be his MSNBC paycheck).