Voters in four key states are choosing candidates to square off in November.
In Colorado, both democratic senate candidates have "presidential" backing. Incumbent Michael Bennet has President Obama's endorsement.
"He's been a breath of fresh air at a time with a lot of hot air," the president said.
While his competitor Andrew Romanoff scored President Clinton's support.
In the republican primary, McCain-backed Jane Horton is vying against tea party favorite Ken Buck.
Minnesota holds a gubernatorial primary while a senate seat is up for grabs in Connecticut. Democrat Richard Blumenthal awaits the winner of Connecticut's GOP primary between former congressman Rob Simmons and former wrestling mogul Linda McMahon.
And in Georgia, Sarah Palin's star power will be put to the test. Her pick in the republican primary for governor, Karen Handel, is pitted against former congressman Nathan Deal.
Some experts say the key to Tuesday's primaries is actually who shows up. Democrats will be paying close attention to see if republican enthusiasm is holding.
A recent Pew research study shows GOP enthusiasm is at a 16-year high. 77 percent of republicans say they will definitely vote this year compared to 65% of democrats.
A big motivating factor – the economy.
"On the national landscape there is one single issue that is dominating and that's the economy,” said CBS News Political Analyst John Dickerson. “Because the economy is so bad, it ends up being a referendum on the president."
Democrats head into November eager to maintain their majorities in Congress but republicans are just as eager to regain that control.
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