To add to dragonwriter's post, there is also the 17th amendment:

"The Senate of the United States shall be composed of two Senators from each State, elected by the people thereof, for six years; and each Senator shall have one vote. The electors in each State shall have the qualifications requisite for electors of the most numerous branch of the State legislatures."

It's exceptionally clear that it's an explicit right to vote for the election of senators.