With primary elections being held in ten states, 8 June is the most significant election day in the US until general election day in November. Today, voters are selecting Democratic and Republican candidates who will face each other on 2 November.
Although typically a smaller percentage of the American public votes in the primaries than in the general election, they will be a key indicator of the reaction to the Democrats after Barack Obama's first year in office. Polls show that there has been a clear backlash against the perception of big government spending, especially after the passage of the trillion-dollar federal health-care bill in March.
The primaries so far have also been a test for the "Tea Party" movement, which seeks to nominate right-wing Republican candidates. In some races, this has led to a split, with the official Republican establishment candidate pitted against a "Tea Party" Republican.
Here are some of the key elections:
An unexpected boost in support for the Tea Party candidate, Sharron Angle, has rocketed her from single-digit support to the lead spot, over the GOP establishment candidate, Sue Lowden.
The Senate majority leader, Harry Reid, the Democratic candidate in the race, has been lagging in the polls, and until recently was expected to lose his seat. The nomination of Angle overnight puts Reid in a much better position in November, as far-right Angle has a smaller war chest and is viewed as the weakest Republican candidate in a primary field of 13 candidates.
The Tea Party movement has excited and mobilised the Republican grass roots. However, if its candidates prove unelectable, the party may die down by November.
The dethroning of incumbents has been a key story this primary season. Two incumbent candidates have been ousted in primary elections so far this year: the Republican-to-Democratic Party-switcher Arlen Specter from Pennsylvania and the Republican Bob Bennett of Utah both failed to gain their party's nomination.
Today, there could be a third upset in the state of Arkansas. The Democratic incumbent candidate, Senator Blanche Lincoln, has been battling the effects of a reverse Tea Party effect. Lincoln has been viewed as too conservative by some Democratic Party supporters, and was forced into a run-off that is taking place on Tuesday. She faced opposition on the left from trade unions after she opposed a key component of the health-care bill.
Lt Gov Bill Halter came out of nowhere to challenge Lincoln in the runoff, after scooping up union campaign money.
In California, the fight for the Republican nomination turned into an expensive duel between a billionaire and a millionaire. The eventual general election winner in November will succeed Arnold Schwarzenegger as governor and inherit a budget deficit of $17bn.
The former head of eBay, Meg Whitman, is seeking the Republican nomination. The billionaire has poured in an extraordinary $71m of her own money into her campaign -- which has cost $80m in total. She is expected to defeat her fellow Republican and former businessman Steve Poizner, a state insurance commissioner, who has spent $24m of his own money.
While Whitman and Poizner are competing to be the most ideologically conservative candidate, another overriding concern is wooing undecided voters in a state that is more liberal than many others in the country.
The Republican nominee will face the 72-year-old Democratic candidate, Jerry Brown, in November. If Brown wins he would be both the youngest and the oldest person to have served as senator for California: he first held the same office 40 years ago.
The story in South Carolina's primary is less about trends and money than plain old dirty politics. The front-runner for the Republican nomination for governor, Nikki Haley, has been accused of infidelity and called a "raghead" by a Republican state senator.
Haley, the first Sikh American to hold state office in South Carolina, is seeking to succeed Mark Sanford, the Republican governor infamous for disappearing for days, before finally admitting he had fled to Argentina and had had an affair.