Whips are often used to 'persuade' MPs in their party to vote the way the Party wishes them to vote.
What power, or powers, do Whips have over backbench MPs to enforce this wish?
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Theoretically, an MP is free to vote however he or she chooses, once elected.
However, a good politician understands that a parliamentary faction is only strong as long as it is united. When an MP votes against the party line on one issue, they can expect that others might follow suit on different issues, especially those where the MP believes very strongly in the party position.
This is also important for the acceptance of the party as a whole. When people voted for party X because their opinion about issue Y, and then a considerable part of the elected MPs votes different, they will rightfully feel cheated.
But when an MP fails to understand these party-strategic concerns, it's the job of the party whip to remind them of two facts:
So while the whip can not sanction a rogue MP here and now, they can threaten that their political career will be over at the end of the legislative period.
There are also anecdotal stories of party whips using more underhanded tactics bordering on extortion or blackmail to enforce party loyalty. But due to the secretive nature of party-internal affairs there is a lot of hearsay and conspiracy theorizing involved in these stories, so I am not going to speculate on how often this really happens.