The state of Missouri has long been seen as a bellwether state, as it had correctly voted for the winner of the Presidency in every election in the 20th century, except 1956, when it voted for Adlai Stevenson over Dwight D. Eisenhower by a small margin of a few thousand votes.
The Missouri tradition of voting for the winner continued in 2000 and 2004, but again, in 2008, by a few thousand votes, Missouri voted for John McCain over Barack Obama.
So only twice did the Presidential winner lose Missouri since 1900, but now it has happened a third time, and this time not only a few thousand votes separate Mitt Romney from Barack Obama.
So the margin is 260,000 votes, a massive lead for Romney, making Missouri clearly lose its status as a bellwether state, at least for the time being!