Though the proposal made headway as the first amendment heard by the Senate's Constitution Committee in about five years, it was unanimously rejected by Senate Republicans during a vote in September.

Grassley opposes

Sen. Chuck Grassley, R-Iowa, said he expressly opposes both amendments and any amendment that would limit campaign contributions by private citizens.

Political contributions are effectively political expression, the senator said, and should be protected as free speech.

"We want to enhance speech, not limit it," Sen. Grassley said. "The more political speech you have, the stronger our democracy will be."

Sen. Grassley warned that drastic changes to the constitution could create a "slippery slope," inevitably leading to infringements on other rights, like freedom of religion and the right to bear arms.

The Senate race in Iowa was the third most expensive in the country, according to the nonpartisan Center for Responsive Politics. For every dollar candidates spent, outside groups spent slightly more than $2.67.

"It's probably an omen for the future, whether for good or for bad, that the day of inexpensive campaigns are over," Sen. Grassley said.