Harkin, along with other senators and a number of liberals in the House, have expressed frustration at the current filibuster rules, which require 60 votes to end debate on most matters before the Senate. They introduced a measure to change the rules on Thursday.
The use of the filibuster has been extended, though, to essentially require the supermajority vote for most provisions, which has led to demands for reform.
Unlike the House, though, which requires the adoption of new rules at the beginning of each term, Senate rules provide that their rules be used continuously unless changed as prescribed -- which requires a 67-vote majority.
Republicans hold 41 votes in the Senate and are unlikely to allow a change to those rules that would diminish their rights as a minority.