Here's why Dark/Electric needs to be the chosen type:
The goal is to wear down Politoed faster than Ninetales can be worn down. How do Sand teams/HO teams often do this nowadays? Band TTar, whose pursuit obliterates offensive Politoed.
This CAP needs to offer similar support to Sun teams. Therefore something with a strong Pursuit (maybe even comparable to TTar's, we'll see when we get to Stats/movepool) is a huge plus. STAB practically is required to acheive that, since Pursuit is pathetically weak otherwise. The simple problem is that Dugtrio/Wobbuffet/Gothitelle are pretty unsatisfactory when it comes to trapping Politoed. An ability alone is not going to cut it.
With Dark/Electric, we can threaten Politoed with STAB super effective moves, potentially STAB Volt Switch if facing Defensive Politoed on a switch to gain Momentum (reinstate Sun most likely), potentially STAB Pursuit against Offensive Politoed to do solid damage. On top of that, if we do decide to give a water absorbing ability, Choice Politoed will be basically auto-pursuited if they take something out with a Hydro Pump.
The key is weather control first, and hazard control second. And an Electric CAP is ideal for hazard control against a Rain team as Tentacruel and Starmie are both weak to Electric moves. Ferrothorn remains problematic, but then... it always is.