But it's a useless threat.
No one here thinks we'll get a deal with the US with the Democratic Party in control anyway. So there's no reason not to proceed with modifications to the treaty with the EU. At least for that reason.
From our perspective, the treaty provides a mechanism to make changes in the event that its implementation causes internal damage to the UK. And that is clearly happening with the Northern Irish government unable to function as a result of the EU's strict interpretation of customs rules.
Why Biden and Pelosi are getting involved in the UK's politics, if it isn't a romantic idea of Irish Nationalism of the sort the Democrats funded (with terrorist tentacles) until 911, is anyone's guess? As much as I admire Pelosi's resistance to Trump, she is naive on foreign policy. For all their sins, the GOP is a more reliable friend of the UK.
I'd certainly be okay with a post-Trump Republican Party from that perspective, even if I dislike their domestic agenda. It works better for us when the US thinks of the UK as an ally. If the Dems think the EU is America's valued friend, note how easily France and Germany folded when Putin breathed on them.