Blunt took his oath with vice president Joe Biden today to make it official.
McCaskill says she's optimistic the new members of the Senate will be willing to compromise.
"Many of the senators that were elected come from states that had previously had Democrats," McCaskill said, "So, obviously these are not bright red places. These are places like Missouri that is very capable of electing both Democrats and Republicans. And I think those members of the Senate are going to be anxious to find compromise and middle ground."
Republicans now have control of the house and more seats in the Senate.