Americans in several states are preparing to go to the polls to decide on party candidates for November’s mid-term elections.
The outcome of these primary ballots will be seen as a key indicator of the volatile political mood in America.
Two sitting Democratic senators are facing serious intra-party challenges, while the conservative Tea Party movement may claim a win in Kentucky.
Polls suggest anti-incumbent sentiment is rife across the US.
"In America today voters don’t like or trust government, business or even the Catholic Church," Republican strategist Mark McKinnon told the BBC.
"The worst thing you can be is an incumbent. The Pope would have a hard time getting re-elected right now."
Veteran Pennsylvania Senator Arlen Specter, who was first elected in 1981, is fighting for his political life against two-term Democratic House Representative Joe Sestak.
Formerly a Republican, Mr Specter switched parties, becoming a Democrat in April 2009.
At the time, Mr Specter said it was because Republicans had shifted too far to the right, but pundits speculated that the switch was motivated by a looming primary challenge from a conservative Republican.
Now, Mr Specter may well lose to a challenger from his left, Mr Sestak, in a campaign where the former’s many years of experience and political weight in Washington have become liabilities.
The latest polling shows the pair in a dead heat.
In Kentucky, two Republican candidates are vying for their party’s nomination for a Senate seat vacated by retiring Senator Jim Bunning.
Kentucky Secretary of State Trey Grayson has been anointed by the Republican establishment, most notably Senate Minority Leader Mitch McConnell and former Vice-President Dick Cheney.
But Rand Paul, son of one-time presidential candidate Ron Paul, has harnessed conservative grassroots energy, mounting a serious challenge to the party establishment’s favourite.
Driven by the burgeoning Tea Party movement, a win for Rand Paul would be the second victory in a matter of weeks for Tea Partiers, who recently helped displace sitting Utah Senator Bob Bennett as the Republican candidate in that state.
Outsiders’ energy
Also on Tuesday, a special election will be held to replace Democratic Congressman John Murtha, who died in February this year.
Mr Murtha’s seat was long considered a Democratic stronghold, but Republican candidate Tim Burns is polling well.
Both parties are viewing this race as an important preview of November’s elections.
They have funnelled over $2m (£1.4m) into the race and former President Bill Clinton recently campaigned there for the Democratic candidate.
"A Republican win in that seat could signal a really big Republican year," Scott Rasmussen, president and CEO of polling firm Rasmussen Reports, told the BBC.
But Mr Rasmussen cautions against over-interpreting the results for any one race. Rather, he says, the results should be looked at in concert.
"You have to look across the board. If Rand Paul wins his primary, if Joe Sestak wins his, and if the Republican wins [John Murtha’s former seat], then the energy really is with the outsiders," says Mr Rasmussen.
And insiders, as far as the public is concerned, appear to include Republicans.
"Rand Paul is really campaigning against Mitch McConnell. If he wins, that will send a clear message that Republicans aren’t immune," Mr Rasmussen says.
"It’s not that people have fallen in love with Republicans suddenly."
But Frank Newport, editor-in-chief of the Gallup Poll, warns Americans not to interpret Tuesday’s results "to excess".
"Every race is idiosyncratic to that district or that state," Mr Newport told the BBC, pointing out that, for example, Arlen Specter’s long history with Pennsylvania voters animates that contest.
In Arkansas, incumbent Democratic Senator Blanche Lincoln has problems on both the right and the left.
Ms Lincoln’s support for the economic bailout and health care reform angered Arkansas conservatives.
Meanwhile, liberals criticise Ms Lincoln for not supporting cap-and-trade energy legislation.
Appeasing the left could help her win her primary, but would certainly lead to a loss in the general election.
Ms Lincoln, like many Democrats in conservative states, has found herself between a rock and a hard place.
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