Set against a backdrop of nationwide frustration with Washington and disapproval of Congress, Tuesday's key Senate primary contests have shaped up as battles of would-be outsiders versus incumbents or the D.C. "establishment."
While it's important to remember these are state-level -- not national -- races with their own dynamics, together they'll add some compelling data points to the political trend lines for 2010.
Here's a few things to watch for on primary day, and beyond:
Pennsylvania: How Has Sestak Made it a Race?
In Pennsylvania's Democratic primary, incumbent Arlen Specter is trying to hold off a hard, late charge from Rep. Joe Sestak, who's closed a big gap in the polls in recent weeks to make this into a dead heat in the Quinnipiac poll released last week.
The trend line here is striking. In early April, this race seemed all but over; Specter commanded a more than 20-point lead in that Quinnipiac poll, had the backing of the president and leading Democratic endorsements, and some questioned why Sestak was even running.
But Sestak's campaign has looked to turn that on its ear and paint Specter as an opportunist who switched parties not out of principle but political calculus. In this environment, that argument may prove especially powerful.
Sestak'smay also find resonance with the Democratic base. (And as a partisans-only primary, it could give that Dem base added influence, and Specter cannot draw as much on independents who may have backed him in the past.)
Meanwhile, Specter's challenging of Sestak's military record doesn't seem to have helped so far; those who do know Sestak have a positive view of him.
Still, Specter (at left) remains the better-known candidate. On Tuesday, watch for whether that ends up helping or hurting him: it usually signals deep trouble that an incumbent who's well-known isn't well ahead at such a late stage.
However, many still say they still don't know enough about Sestak. If he falls short, it may be that he ran out of time or resources to get his name and message out.
And watch for whether Specter can still pull it out with organizing -- if he can turn out his voters, especially in and around Philadelphia -- and whether the president's backing can help with that.
Arkansas: Can Lincoln Survive Now...And November?
Down in Arkansas, embattled incumbent Democratic Sen. Blanche Lincoln (at left) faces a challenge of her own from Lt. Gov Bill Halter. Halter is helped by labor and spurred by anger over Lincoln's perceived ties to corporate interests. And the bailout remains an issue, here as elsewhere.
Though some polls suggest Lincoln is on track to finish on top Tuesday, watch for whether she (or Halter) can get enough to avoid a runoff in this multi-candidate race -- which would mean more weeks spent on each other and still less time focused on the challenge for the fall.
In the bigger picture, Lincoln has quite the balancing act: she needs to not just ultimately hold off this primary challenge but also must eye November in this increasingly Republican-leaning state. She's looked to stay viable and centrist for the general election. Watch for whether her recent support for financial reforms proves to be pushback enough against Wall Street and the ways of Washington for Tuesday's voters.
Still, this primary process raises questions about troubles ahead for her even if she ultimately survives. Without strong -- in fact, near unanimous -- support from the Democratic base, which she obviously does not have right now, winning in November becomes that much harder.
Kentucky: More Power for the Tea Party?
On the Republican side, Kentucky's Senate primary pits Tea Party favorite Rand Paul - an ophthalmologist, political newcomer and son of Rep. Ron Paul - against Secretary of State Trey Grayson.
Grayson carries the backing of some of the party's top national leaders --, and Dick Cheney in particular. Sarah Palin, among others, is backing Paul.
Polls here suggest an edge for the newcomer Paul heading into the election. One key to watch will be turnout and organizing, how well Paul's get-out-the-vote operation matches up against Grayson's, and whether activists' enthusiasm for Paul drives turnout to compensate if it doesn't.
Tea Party organizers claimed a major impact over the Republican nominating process in Utah last week, as incumbent Bob Bennett failed to even emerge from the state convention. Kentucky would be a second triumph if Paul succeeds, and a more widespread test in a larger electorate.
But afterward, if Paul does win, watch for how his campaign is perceived by the moderates he'll need to ultimately win the general election. If he happens to end up in the national spotlight come the summer or fall, that question could also apply not just to Kentucky but to the GOP's courting of moderates nationwide.
One final note for all three: As with most primaries, turnout is not only the deciding factor but often the hardest to one predict. Gauging the likely electorate isn't always easy especially when states haven't seen competitive midterm primaries in a while, and there isn't a comparable history to use as a guide.
Anthony Salvanto is CBS News Elections Director. You can read more of his posts in Hotsheet here.