In Newberry v. United States, a Senate candidate in a state primary election challenges the constitutionality of the Federal Corrupt Practices Act. The candidate had been convicted of violating federal limits on the amount of money that could be used in primary and general elections.
The Court notes that although the 17th Amendment changed who elects senators (from state legislatures to voters in each state), it did not modify Article I, Section 4 of the Constitution. This provision gives states the power to determine the time, place and manner of holding Senate elections, unless Congress alters state rules. Here, where Congress has specifically spoken, the federal rules are permissible.