One major theme to watch for as primary returns start rolling in for Tuesday's key Senate races: Is incumbency all it's cracked up to be?

In most election cycles, holding a seat in Congress is a huge tactical advantage, since sitting lawmakers are usually able to draw on the campaign war chests

that national party organizations and congressional campaign groups build up — in addition, of course, to all the time that elected officials spend

fundraising on their own.

That advantage shrivels, though, when the electorate seems to be in throw-the-rascals-out mode, as seems to be the case so far in 2010. Rather than coasting

into a lavishly funded media push in the homestretch of their primary races, two veteran senators — Arlen Specter of Pennsylvania and Blanche Lincoln of

Arkansas — are fighting for their political lives. In Coach outlet Kentucky, the battle for the seat opened

by Republican Jim Bunning's retirement is between a candidate endorsed by the state's GOP hierarchy, led by Senate Minority Leader Mitch McConnell, and the

self-described tea party candidate, Rand Paul. Tuesday's ballot may well signal the depth of popular anger at Washington — and might serve as a bellwether

for which party ultimately gains control of Congress in November. Here's a quick primer on the big three Senate races:

Pennsylvania

When Arlen Specter switched parties a year ago, he made no secret of his main motive: political survival. Facing a nasty GOP primary against onetime opponent

Pat Toomey, Specter wasn't sure he could win. In any other case, such candor might help a candidate, but not this year. Even though Specter often siphoned

off some Democratic support in his previous Republican campaigns, the longtime senator has struggled to convince members of the Democratic base that's he

really one of them. In recent weeks, Joe Sestak, a Democratic congressman from the Philadelphia suburbs, has closed the gap with Specter, earning

endorsements from groups like the abortion-rights group NARAL and MoveOn.org. And Sestak has been savvy not merely in questioning Specter's party loyalty,

but also in joining the chorus of other primary New Coach Handbags

challengers denouncing incumbent politicians and campaigning for a "new generation" in Washington. To counter that argument, Specter has been highlighting

his "experience" in his own campaign messages. That's not just a dig at Sestak's limited time in Congress, but also a reminder of Specter's long resume in

the Senate. Unfortunately for Specter, that may not be much of a selling point for voters Tuesday.

Arkansas

Heading into the 2010 cycle, Blanche Lincoln was considered a safe bet for re-election. She was a centrist Democrat — a position that played well in

Arkansas, a once-liberal state that has increasingly swung right in recent years. But Lincoln's noncommittal positions on big-ticket legislative issues like

health care reform earned her the ire of liberal activists and labor unions, including the deep-pocketed AFL-CIO, which began spending money on attack ads

calling for her ouster. On Tuesday, she faces Lt. Gov. Bill Halter, who, in spite of getting endorsements from groups like MoveOn.org, says he's not

challenging Lincoln from "the left" but more as Coach Backpack Bags an

outsider — an argument that has traction in this anti-incumbent year. Though polling has been limited, a recent Daily-Kos/Research 2000 poll found Lincoln

leading Halter by 9 points, 46 percent to 37 percent. And the race might not be over Tuesday: If neither candidate cracks 50 percent, a runoff would

automatically be scheduled.

Kentucky

It's not just Democrats facing spoilers in their own party. When McConnell anointed Secretary of State Trey Grayson as the GOP establishment candidate to

succeed Bunning, Republicans thought the race was over. Not so much. In recent weeks, Rand Paul — the son of libertarian Texas Rep. Ron Paul, an insurgent

GOP candidate for president in 2008 — has taken a double-digit lead in most surveys. Rand Paul has made his insurgent primary bid a referendum on

Washington, tapping the energy of the tea party base. He criticizes Republicans as much as Democrats and has notably refused to say whether he would even

support McConnell for a role in Senate leadership should he win. While Grayson was endorsed by Dick Cheney and other key Washington Republicans, Paul picked

up a major endorsement of his own: Sarah Palin. In recent days, McConnell, while not quite distancing himself from Grayson, has sought to make nice with

Paul. In an interview on Sunday's "Meet the Press," McConnell insisted that the open primary was a good thing for the party — though he insisted a Paul

victory had nothing to do with establishment Coach Hampton Bags politicians

like himself.