Will not work
Russians were significantly smarter than that. They did not buy ads supporting Trump directly. They bought ad space to promote organizations with socially divisive agenda and/or promoted various causes to increase social division. Have a look at the transcripts of the Facebook, Google and Twitter congress hearings. So any of the McCain proposed restrictions will not work.
The Russians have [analysed | guessed | who knows] that the biggest damage they can cause to their adversary is by promoting social division and instability. What they are doing is not about getting "their candidate elected" - it is about maximizing the damage. The candidate is usually just the means.
It is also clearly, purely and openly by policy in retaliation.
Officially, this has become their policy 12 years ago in 2005 (or was it 2006? - two weeks after one of the Bush visits) after the Kondoleeza Rice letter telling them to f*ck off and that USA will invest into any political entity in Russia and around their borders as they see fit as this is "American Sovereign Right". While the letter is classified on the USA side, Russians have leaked excerpts for a long time and I think you can get your mitts on a copy of it in their national archive.
Historically this fits - there is little or no evidence of Putin sponsoring anyone in Europe, USA, etc prior to that. After that we got a whole raft of strange agenda organizations with unclear financing sources sprouting up (as a first instance), followed by the referendum, USA election, elections in Eastern Europe, etc.
We should have de-escalated this long ago. Instead of that we are pouring more and more billions in attempted regime change in Russia and around it. That did not work and is not likely to work any time soon. At the same time they are doing more damage to us than we are doing them.