I'd say 20% maximum. I know there were theories that he could reach up to 30% but that probably means that more would vote for him as for the Republican Party and I doubt that so many Americans would vote a candidate above the party they like.
Don't forget that many Republicans help him all the time when he has his stupid moments (so every day). If he would be independent, they would also go against his stupidity and wouldn't spin it for him just to have arguments against the Dems.







