Increasing membership of the House has the effect of reducing the advantage less-populous states have (both in the house and in the electoral college). That might make it politically infeasible to pass.
Probably true. But I would hope that argument could be deflated easily, since it was the senate that was supposed to give smaller states a disproportionate amount of influence.
Which is why people who don't want to pass it for this reason will state some other reason, like "5000 representatives is obviously silly and way too many"
