is being transported.

If the funds for transportation are paid out of a PAC or other third-party whose intent is to transport only voters supporting a particular party, that can be construed as a form of bribery.

If the transportation services are predominantly available to voters in a specific area or who are affiliated with a specific party vs. another, then isn't that a form of suppression too?

The only way this works is if (1) the funds are being provided by a completely independent source and (2) the services are equally provided to voters regardless of their affiliation.

I'm not suggesting these are "the" concerns, but these are things I would think of that are fairly legitimate.