On top of the fact that it's practically impossible to make a libel accusation stick when political speech is even barely provable - being incorporated doesn't prevent a libel proceeding from moving forward. It just means that it's more difficult to pin it to individuals within the corporation.
Even then, there are ways that the corporate veil can be pierced to still go after individuals.
If your corporation commits libel, you are not liable.
That's one problem.