I can see the Congressperson in question beginning with no seniority in the party but I don’t see how they would lose seniority in Congress.
In what states would they be subject to recall? AFAIK, the premise is they’re the “best person running for the office and thus were elected as such.”
That’s right. They may lose party seniority, but not Congressional seniority. However, with the extreme partisanship currently prevalent on Captol Hill, it’s quite likely that the other party would make an offer to retain seniority as an inducement for a congressperson/senator to jump parties.
There’s no provision in the US Constitution for members of Congress to be recalled, therefore they can’t be recalled. It doesn’t matter if the state they’re from has recall for state offices – that doesn’t apply to federal offices. They can lose their office either by resigning or by being expelled by the House they’re a member of.
The House of Commons ‘factsheet’ on the Chiltern Hundreds does a good job of explaining why historically the Commons wasn’t keen on the idea of MPs resigning mid-Parliament.
There may be some regional variance in the term, then - here in Canada it does mean moving from the Government side to the Opposition side (or vice versa) on a permanent basis, not just on a particular vote.
It may be that the difference stems from the voting system used in a particular House. In the UK, the recorded votes in the Commons are done by the members physically dividing and being counted, so a member does have to cross the floor to vote against his/her party.
But in the Canadian House of Commons (and all of the provincial Assemblies, as far as I know), recorded votes are done by the members standing at their seats and being recorded by the clerk - who calls out their riding name, not their own name, and they then sit down. A member of the government who wants to vote against a measure can do so from the comfort of his/her own seat, without physically crossing the floor.
As mentioned, it has no effect in Canada; an MP (or MLA provincially) can and often has been booted or switched parties. Generally, like the US, a riding is tilted to one party or another, so changing parties has a good chance of costing you your seat. However, you’re set until the next election unless expelled by the house - usually for criminal misdeeds.
Plus, the head of the party (Prime Minister or opposition leader or whatever…) has final say over whether you can file papers next election as representing that party. So you can be nominated by say, the Conservative riding association but be rejected by the Conservative headquarters. Of course, a party that fights the will of the local grassroots is probably writing off that riding, but sometimes the views or actions of the candidate have to be rejected - if they make racist statements or buck the party line, for example.
One candidate in the Canadian NDP was booted from the party for expressing opposition to abortion (being Catholic, IIRC). She then ran as an independent, split the vote with the official candidate, and allowed a different party to win. Another was denied a Reform nomination won as an independent, and later offered inducements as he was dying of cancer to join their party for a crucial vote.
Another problem is that the central parties (who wrote the laws) control the purse strings. Donations to political parties are hugely tax-deductible for the first few hundred. However, except during the actual election this money generally only can go to the central party HQ who can dole it out as needed.
Another tactic, famously used by Jean Chretien in 1993, is to bypass the grassroots. The party HQ can “parachute” a candidate into a riding. Chretien also famously used the “3-hour nomination window” where the people who wanted to run has 3 hours from the notification being passed by the tame captive grass-roots association to get their papers in, signed by a few hundred voters. This because Chretien wanted to reward cronies who supported him, even though they were relatively useless MP’s and in the coming election, the Liberals were sure to win so everyone else wanted in on the action.
So, once the party boots you or you leave, it is difficult to get re-elected but not impossible.
The OP may be wondering about places that have proportional representation, as a previous post mentions. In that case, the voters have voted for the “party list” so if you aren’t on the list any more, why should you stay in parliament? Canada does not have any PR, nor Britain, so I guess that does not apply.
The problem with PR is that the modus operandi of the political animal is therefore no longer grass-roots schmoozing, but central party office politicking. The figurehead leader(s) go out and get the vote, and the people that want the cushy MP jobs and salaries just schmooze the party brass to get high enough on the list to get a seat. As a result, you are more likely to have as MP’s those dipwads who used to play politics on the college student council, than the people who get into politics at the grasssroots level to make changes to the world. Grassroots-selected members turn out badly often enough. Imagine leaving the choice in the hands of some party Central Committee.