Speech is absolutely power. Look at the power the press wields: "the pen is mightier than the sword".
Let's not confuse transactional speech with normal speech of every day citizens. One can give a speech for money, that's transactional. There becomes something for something, labor for something. You buy a newspaper, you purchase information. Here, "speech", is being reduced to a product being purchased.
You influence with speech, but you can't purchase power like you can with money.
There is a difference. Speech can be powerful persuasive, but money is POWER. It's the difference between talking about cars, and gasoline.
Talking about cars is or can persuasive, but gasoline is stored power, real power.
Nothing corrupts more than money. Money should not be ruled as speech, just for that reason alone.
One person has a billion dollars, another person has a hundred dollars.
One person can talk to a congressman to persuade him, his influence is just as good as the next man's.
But, if person donates a million dollars to a congressman, when another person has only 100 dollars, that's an unfair advantage.
Speech should be equal. One man, one vote. But money is power and gives the giver undue influence.
That is why we should take money out of politics. We should not call money speech simply because it's the just thing to do.