Udall spokeswoman Jennifer Talhelm said this week that Sens. Jeanne Shaheen of New Hampshire and Ron Wyden of Oregon signed on to his proposal.
Udall, Shaheen and Wyden are Democrats.
Talhelm says others have also expressed interest.
Udall says the tradition of having the parties sit on opposite sides emphasizes differences and divisions.
Third Way, a group advocating moderate political ideas, also proposed mixed seating as a show of unity. Udall is an honorary chairman of the group.
President Barack Obama will deliver his State of the Union speech Jan. 25.
(Copyright 2011 by The Associated Press. All Rights Reserved.)