It's covered by the 1st amendment, which enumerates the Right of the people to petition the government.
Anyway, the Constitution doesn't have to say it.
This is covered by the 9th amendment which states that if the Constitution doesn't delegate a power to the federal government, that power belongs to the people.
I fail to understand how campaign contributions or advocacy ads to assist the election of a candidate are petitioning the government. I petition the government through a call,letter or personal meeting witht he person or person whom I wish to address. My access should not be limited or enhanced by the zeros after the check I just wrote to assist that particular person. That's the perversion of the process.