WASHINGTON -- In one of the most unpredictable political seasons in recent American history, primary elections Tuesday in three states will test the mettle and staying power of congressional incumbents, the strength of ultraconservative tea parties and the unity of the Republican Party.
The ballots cast in the primary contests will determine which candidate represents the Republican and Democratic parties in the November general elections for all seats in the House of Representatives, some Senate seats and in a number of state governors' races.
The midterm contest, so called because it falls halfway through of the president's term, historically benefits the party that lost the last race for the White House.
That should give Republicans a leg up this year, but the minority party is riven nationally by internal challenges from the largely right-wing tea party groups that have sprung up like mushrooms after a spring rain.
Tea party successes in the primaries could cause middle-of-the-road independent voters to turn away from Republicans in November.
The primary balloting in Pennsylvania, Kentucky and Arkansas are viewed as equally important for what they may portend for the general election later this year -- a reflection of the mood of the country as it struggles to emerge from the deepest economic downturn in at least 70 years.
On the eve of the busiest primary night of the year so far, White House spokesman Robert Gibbs said Monday that President Barack Obama was following the races, but "not that closely."
"We have supported incumbent Democratic senators and we've done a lot on behalf of each campaign," he added, referring to Arkansas Sen. Blanche Lincoln and Pennsylvania Sen. Arlen Specter.
It was not clear what impact Obama's involvement would have on the incumbents, who are under extraordinary political pressure in a year of well-documented voter dissatisfaction with Washington.
In Pennsylvania, the 80-year-old Specter quit the Republicans and joined the Democrats a year ago, drawing the support of Obama, organized labor and Pennsylvania Gov. Ed Rendell as he set out to win a sixth term, his first as a Democrat.
But Rep. Joe Sestak, a former Navy admiral, got some last-minute momentum with a television commercial that showed his rival accepting a glowing endorsement from Republican President George W. Bush in the 2004 campaign and saying he had switched parties so he could win re-election. Late polls showed a highly competitive race.
Former Rep. Pat Toomey, supported by the anti-taxation Club for Growth, campaigned as the prohibitive front-runner for the Republican nomination, six years after losing to Specter in a Republican primary.
In Kentucky, Republican Senate Minority Leader Mitch McConnell made a late television commercial on behalf of Secretary of State Trey Grayson, who is battling Rand Paul, darling of the grassroots conservative tea partiers. Paul is leading handily with their backing and that of former Alaska Gov. and vice presidential candidate Sarah Palin and retiring Sen. Jim Bunning.
Grayson had support from the state's Republican establishment. Paul is riding tea party discontent, complaints about unchecked government spending, excessive taxation and a perceived dangerous spread of governmental influence throughout American life.
The movement's name is taken from the 1773 Boston Tea Party, a protest in which activists in the then-British colonies in America boarded ships and threw their cargo of English tea into Boston Harbor in a symbolic protest against taxes.
Among Democrats, Kentucky's attorney general, Jack Conway faces Lt. Gov. Daniel Mongiardo.
In Arkansas, incumbent Democratic Sen. Blanche Lincoln faces a challenge to her hopes for a third term from Lt. Gov. Bill Halter, who has union support in protest of the sitting senator's votes on health care, trade and union organizing.
Late polls showed Lincoln ahead in a multicandidate race, but far from certain of the 50 percent threshold needed to avoid a runoff.
Rep. John Boozman was the acknowledged Republican front-runner for the Senate nomination for a seat that the Republicans hope to win in November.