Why is it freedom of speech to give large sums of money to a political candidate. That's certainly not in the constitution, aren't conservatives originalists?

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Yesterday, John McCain actually said that if he’s president he’ll take on, and I quote, 'the old boys’ network in Washington.' I’m not making this up. This is somebody been in Congress for 26 years, who put seven of the most powerful Washington lobbyists in charge of his campaign. And now he tells us that he’s the one who’s gonna take on the old boys' network,” he said. “In the McCain campaign that’s called a staff meeting!- Obama, 9/17/2008