WASHINGTON � US voters cast ballots Tuesday in a handful of key races that may foretell how anger at Washington will shape November elections that will decide the future of President Barack Obama's agenda.

Three primaries are expected to test the limits of Obama's influence over the Democratic party's rank-and-file, and the Republican party's ability to harness an internal insurrection from arch-conservative "Tea Party" activists.

And one special election will shed light on Republican abilities to rally their motivated core supporters to capture conservative districts, key to the party's hopes of retaking the House of Representatives in November.

Those factors are all expected to decide the result of the mid-term elections -- when the sitting US president's party historically loses seats -- which could scuttle key Obama initiatives like overhauling US immigration.

In Arkansas and Pennsylvania, two embattled veteran Democratic senators endorsed by Obama, Blanche Lincoln and Arlen Specter, face upstart candidates backed by the party's more left-leaning core supporters.

On Monday, White House spokesman Robert Gibbs played down the importance of the primaries, which select each party's candidate for November, saying Obama had followed those races "not that closely" despite helping both campaigns.

In Kentucky, the Republican Party establishment also faces an upset, with polls showing Republican Senate Minority Leader Mitch McConnell's anointed candidate running behind a "Tea Party" favorite.

The three battles come after two powerful long-serving lawmakers, Republican Senator Bob Bennett and Democratic Representative Alan Mollohan, became high-profile casualties of a tide of anti-incumbent mistrust.

In a conservative Pennsylvania House district, Republicans and Democrats will square off for the seat held by the late Democratic Representative John Murtha for 36 years, a race with broad national implications.

Republicans plotting to recapture the House have set their sites on districts, like Murtha's, held by Democrats but captured by the Republican candidate in 2004, 2008, or both.

Defeat could spell trouble for Republicans, who have crowed over a series of polls that show their voters are more energized about the November battle for all 435 House seats, 36 Senate seats, and 37 governorships.

Recent national opinion polls have found the US public split on which party should control Congress, but just one in three respondents wants to send their lawmaker back to Washington -- the worst climate for incumbents since 1994.

"That's the national attitude," said Specter, who has also been bedeviled by his April 2009 decision to quit the Republican party and become a Democrat. "I have fought the bickering and the partisanship in Washington," he told CNN.

Representative Joe Sestak, Specter's rival, said last week that Pennsylvania Democrats "do not want to send back to Washington a career politician who will pursue the broken deal-making" that soured them on politics.

Recent polls show Democrats Specter and Sestak, a retired vice admiral, running behind Republican hopeful Pat Toomey.

In Kentucky, Republican voters appear set to hand "Tea Party" darling Rand Paul the party's Senate nod over McConnell's hand-picked favorite, Trey Grayson, Kentucky's secretary of state.

Paul -- whose father, Republican Representative Ron Paul, ran for president in 2008 -- has openly capitalized on disaffection with Washington, repeatedly urging voters to "send some different Republicans" to the capital.

McConnell, eager to avoid an internal schism that could harm the party's prospects in November, told NBC television Sunday that Republicans would hold "a unity rally" in order "to get behind the winner and win in November."

In Arkansas, Lincoln enjoys establishment support, an edge in polls, and a vast money advantage over Lieutenant Governor Bill Halter, whom the party's labor union allies support.

Lincoln has hitched her political fortunes in part to a sweeping bill to reform the rules on Wall Street, pushing for tight curbs on trades in arcane financial instruments called derivatives blamed for fueling the 2008 global economic meltdown.

Democrats face an unusual problem in November: Sweeping victories in 2006 and 2008 gave them control over nearly 50 swing districts that could now tilt the other way, handing Republicans the House.