Which party benefits electorally from Breyer's retirement?

[kwabbit]

Probably Democrats. The Kavanaugh battle energized Republicans in 2018 and potentially allowed them to flip MO, IN, and ND.

If the Republicans were wise they’d silently oppose the nomination and hope for Democratic infighting to sink the nominee. If the Democrats are unified, then don’t try to fight the nomination, just let it pass through and it won’t affect the midterms. If they fight it too hard, the nominee will still make it onto the court but then you will have fired up the completely unenthusiastic Democratic base.

There might be enough acrimony between Sinema or Manchin and the rest of the party where it could become ugly again. Sinema in particular seems to not care about being on good terms with her colleagues and the attacks on her have been vicious of late.