Pittsburgh ? Whoa there. The story line going out across the country — that the public’s fury against politicians is being whipped up, and then reaped, by the tea party movement — might be missing the bigger picture. Look at Pennsylvania’s Democratic senatorial primary and you’ll see a different story entirely.

Right now it’s anyone’s guess whether longtime Sen. Arlen Specter, a party switcher of Churchillian magnitude and maybe amplitude, survives a primary challenge Tuesday from Rep. Joe Sestak of the Philadelphia suburbs. But the tea party has nothing to do with this fight, one of the most bitter in the country. This is a fight among Democrats.

So, too, is the primary struggle that reaches a Tuesday climax in Arkansas, where Sen. Blanche Lincoln is trying to fend off a surge from Lt. Gov. Bill Halter.

Then look at Colorado, where appointed Sen. Michael Bennet was defeated in March party caucuses and is bracing for another blow at the state party assembly Saturday in Broomfield, when Democrats decide whether Bennet gets preferential ballot position in a blistering primary three months from now.

None of these endangered Democratic incumbents can be comforted by last week’s primary defeat of Rep. Alan Mollohan, who had represented the northern part of West Virginia for 28 years and whose father held the seat previously. He lost to state Sen. Mike Oliverio. It wasn’t even close.

Earlier this month, Sen. Bob Bennett of Utah became the first victim of the anti-incumbent rage of 2010, losing the Republican nomination in his re-election campaign. Sen. John McCain of Arizona, who faces a challenge from former Rep. J.D. Hayworth, is another endangered Republican entering a bloody party primary.

Fire burning on right — and left

But the key to understanding this year’s elections is seeing that the fire — which also consumed Gov. Charlie Crist of Florida, who abandoned his efforts to win the GOP nomination for Senate and is instead running as an independent — isn’t only on the right. It burns on the left, too. Indeed, of the five challenges to sitting senators, three involve Democrats.

In a normal year, no senator faces a renomination fight. No senator, for example, faced a credible challenge in 2008. Only one did in 2006, when Sen. Joseph I. Lieberman of Connecticut won re-election as an independent after having been defeated in a Democratic primary. And only one did in 2004, when Sen. Specter, then a Republican, beat Rep. Pat Toomey by only two percentage points. (Toomey, by the way, is certain to be the Republican Senate nominee this year.)

So it truly is unusual to see five senators facing primary challenges.

All around us is impatience, even disgust, with sitting politicians, and the rebellion against incumbents is a middle-class and mainstream movement, no less pervasive than Facebook. Polls bear that out. A Rasmussen Reports national telephone poll less than a month ago showed that fully 11 percent of Americans — your eyes are not deceiving you even though your mind will not quite be able to comprehend this — rated congressional performance as excellent or good.

No grade inflation among the public this year. No mercy, either.

“The unhappiness out here is much, much greater than some Republicans unhappy with Barack Obama,” says Peter Brown, assistant director of the Quinnipiac University Polling Institute. “It is pretty broad, it’s pretty deep. Pennsylvania is not a state dominated by conservative activists, and yet the Democratic incumbent could lose.”

Specter vulnerable

Specter’s greatest danger: One voter in seven is undecided … about a man who has held his Senate seat for 30 years.

Every political system has its own idiosyncrasies — we’ve just been given a bracing reminder of that in the British elections — but consider this homegrown one: Voters find members of Congress, increasingly even their own, repellent. Still, with the exception of the House primary in West Virginia, the challenge to these incumbents is coming from the periphery. It is not too much to say that this year’s political rebellion is fueled by fury on the fringes.

In all five cases where senators face difficult primaries, the challenger is a candidate of more — more liberal (Pennsylvania, Arkansas, Colorado) or more conservative (Arizona, Utah). The outsider wants more taxes (Democrats) or more budget cuts (Republicans).

Sometimes it is a matter of degree — anywhere except Arkansas Halter wouldn’t be considered more liberal than anyone — but the degree makes a difference. And in each case, the challenger will be helped by low turnout. This is more than a characteristic of these primary elections. It is a necessary condition of them.

Alienated electorate

Sure, everybody’s alienated. But the disgust with politicians runs so deep that many voters will stay home here in Pennsylvania on Tuesday and in other primary venues throughout the spring and summer. But the activists, who are the oxygen of these challenges, will not stay home, and that is the danger that Specter and Lincoln face Tuesday and that the others face as the year wears on.

“It’s an accumulation of things, but the public was promised a lot of change. Washington still works the way it always did, and the change people have been given hasn’t made them happy,” says Jennifer Duffy, the savvy political handicapper at The Cook Political Report.

Who’s to blame? That hardly matters. The practical among us believe the public will blame the incumbent. The analytical among us believe the entire elected political class missed the lesson taught in their own classrooms — the elections of 2006 and 2008.

Congressional Democrats didn’t get the message of the 2008 election when Barack Obama talked about a post-partisan politics (only to win his health care bill on a strictly partisan vote). But then, the Republicans didn’t heed this message either and failed to recognize after the 2006 election that there was a reason they were forced to relinquish control of the House they had possessed for a dozen years.

For people whose principal skill is supposed to be their keenness in evaluating public opinion, American politicians resemble a confederacy of dunces.