It's not the end of the fight, but it's great to see that the efforts are working! I sent a handwritten letter to my MPs a few weeks ago about this issue but no answer so far...
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Yes, writing letters to these people is unlikely to help. The only language they speak is in votes. They have to be convinced that they will lose reelection over the issue. A conditional prediction market for their reelection given they vote a certain way would be the most effective tool.
The first step really is just getting the politician to think about what they're voting on.
They also don't actually necessarily get _that_ many letters.