So the thing about Kyrsten Sinema, just like John McCain before her, is that Arizona has open primaries, where a large share of party-unaffiliated voters can vote in whichever primary they want. Primaries are the primary way parties can discipline their elected representatives – you last decade saw Labour parliamentarians in the UK fight off “reselection” primaries for incumbents because they knew they were rightward of their (harder core in the UK, where narrower paid party membership reflects active interest affiliation) local parties – and in this setup a high national profile, incumbent advantage, and appeal to opposite-party “leaners” (on the basis of shared moderation or cutting losses) drawing them into a party primary experienced (and campaignable) as a general civic referendum can represent a counterweight.