(WASHINGTON) -- The magic number for Republicans in 2012 is four.
The GOP has to pick up just four Senate seats in order to gain control of the upper chamber, making this the fourth straight election where the balance of Congressional power lies in just a handful of races. And while the election is still more than a year away, the battle lines are already starting to be drawn.
A total of 33 seats will be up for grabs in the 2012 election, 23 of which are held by Democrats. Republicans will have open opportunities in six of those 23 states where the Democratic incumbent is retiring.
Montana, Missouri, Nebraska, and North Dakota are shaping up to be some of the closest Democrat-defended races, with Nevada and Massachusetts presenting possibilities for a switch from red to blue.
Only two Republicans are retiring, Kay Bailey Hutchison of Texas and Jon Kyl of Arizona, although the GOP will likely hold on to those traditionally red-state seats. With Tea Party popularity high in many states, GOP Senators could face tough primary races.
The still-floundering economy could mean trouble for Democrats who have twice as many Senators up for re-election than Republicans. But the extra attention and resources of the Obama campaign is expected to turn out much larger numbers of Democratic voters than went to the polls in 2010, when Republicans gained six seats in the Senate -- but saw a decisive victory in the House of Representatives, wresting an historic 63 seats from Democrats, breaking that party's majority.
Copyright 2011 ABC News Radio