Even in the most anti-incumbent primary season of the past few decades, less than 5 percent of members of Congress lost their primaries.

Rep. Jean Schmidt is sworn in in January 2011. (Susan Walsh/AP)

But relatively speaking, this looks like one of the most anti-incumbent years in decades. There are several factors in the coming election that will lead to an increase in the number of members sent home early — and it’s quite possible we could see more incumbents lose than at any point in the last 40 years.

The first factor is redistricting. This has both led to 11 primary matchups between incumbents (which means at least 11 incumbents will lose their primaries) and has given plenty of members plenty of new territory to deal with.

Second is the mood of the electorate. Congress’s approval rating continues to hover around a record low, and uncertainty about the economy has created an American electorate with an itchy trigger finger.

Over the last three elections, though, voters have made their statement by sending incumbents home in the general election, not the primary.

Third is the advent of outside groups. First it was the tea party and conservative groups like the Club for Growth, and now it’s an anti-incumbent super PAC asserting itself. The Club is going big against Sen. Richard Lugar (R-Ind.) in his primary, and the Campaign for Primary Accountability is promising to spend millions trying to take down incumbents of both parties in their primaries — and in the process giving primary challengers something they often find in short supply: financial support.

That said, the idea that the 2012 primary season will be a bloodbath for incumbents is foolhardy.

Four states have held their congressional primaries so far, and incumbents have won 43 of the 44 races in which they’ve taken part. Rep. Jean Schmidt (R-Ohio) was the lone exception, while Reps. Dennis Kucinich (D-Ohio) and Don Manzullo (R-Ill.) lost to other incumbents.

But there is anecdotal evidence that incumbents are taking less of the vote so far than they usually do. And some expect that to start translating into increased losses for incumbents.

Over the last 40 years, the worst year for incumbents was 1992, when 19 House incumbents and one Senate incumbent fell.

Since at least 11 House incumbents will lose primaries to other incumbents, that means nine more would have to fall to set the new standard.

Former congressman Tom Davis (R-Va.) said that this year is more of a mix than 1992, when the losses were driven by redistricting..

“There’s more voter unrest today but less redistricting challenges than ‘92, when the Voting Rights Act brought huge changes,” Davis said.

Former congressman Martin Frost (D-Texas), himself a victim of redistricting changes in 2004, said that unrest is palpable. As for its effect on primaries, though, the jury’s still out.

“The public is very upset with Congress for good reason,” Frost said. “I have never seen as much cynicism.”

If this year does exceed 1992’s carnage, it starts with the 10 races listed below: Our top 10 incumbents who could lose their primaries.

(For the purposes of this exercise, we’re excluding the incumbent-versus-incumbent matchups.)

To the line!

10. Rep. Walter Jones (R-N.C.): If there’s one guy who’s ripe for a conservative primary challenge, it’s Jones. Jones frequently votes against his party on the major issues of the day, the most recent being Rep. Paul Ryan’s (R-Wis.) budget. In fact, it’s a wonder he’s escaped serious opposition in recent years. That could change this year, with former New Bern Police Chief Frank Palombo running against him and the Campaign for Primary Accountability targeting him. We’ll see if Jones is actually threatened, but he’s a ripe target.

9. Rep. Gary Miller (R-Calif.): Miller appeared to have caught a break when Rep. Jerry Lewis (R-Calif.) announced he would retire. Rather than challenging Rep. Ed Royce (R-Calif.) in the Orange County-based 39th district, Miller shifted inland to run for Lewis’s 31st district. Problem was, he ran into state Sen. Bob Dutton (R). Both men will take part in California’s new “top two” primary in which the top two candidates in an open primary — regardless of party — make the general election. But in this Democratic-leaning district, there’s likely only room for one Republican in November.

8. Rep. Roscoe Bartlett (R-Md.): The Maryland press has focused on the nasty Democratic primary for the state’s newly-competitive 6th district. But Bartlett is getting battered by seven rivals, who all agree he isn’t ready for what could be the toughest House race in the country. State Sen. David Brinkley and state Del. Kathryn Afzali are both strong contenders. But if they all split the vote up too much, Bartlett will survive.

7. Rep. Ed Towns (D-N.Y.): Queens Democrats have backed Towns for the newly redrawn 8th district, but he still faces primaries from state Assemblyman Hakeem Jeffries and City Councilman Charles Barron. Barron held Towns to 47 percent of the vote in a 2006 primary, but the up-and-coming Jeffries, who has the support of the Working Families Party along with some influential state legislators, may be as big a threat. After some back-and-forth, the final redistricting map leaves Jeffries’s political base mostly intact, so Towns still has reason to be worried.

6. Rep. Tim Holden (D-Pa.): Holden was one of few Democrats who survived in a conservative district last election, and Republicans appeared to do him a favor by moving many of his Republicans to shore up neighboring GOP incumbents and giving him a much more Democratic district in the process. But that new territory also drew new intra-party opposition, and lawyer Matt Cartwright has reportedly put together $600,000 for his primary challenge April 24. Cartwright has also gotten the support of some liberal groups against the Blue Dog Holden, and the Campaign for Primary Accountability has said it will spend $200,000 on taking down Holden.

5. Rep. David Cicilline (D-R.I.): The freshman and former Providence mayor doesn’t officially have a big-name challenger yet, but polling shows he’s severely unpopular, and 2010 opponent Anthony Gemma is expected to enter the race any day now. Gemma finished second to Cicilline in an open primary in 2010, losing by 14 percent, and the Democratic establishment has rallied behind Cicilline (independent Gov. Lincoln Chafee also just endorsed him). But it’s hard to ignore numbers like these: A recent Brown University poll showed just 15 percent of Rhode Islanders rate Cicilline’s job performance as good or excellent. Another 24 percent say he’s done a “fair” job, and 43 percent say he’s done poorly. Not good.

4. Rep. Fred Upton (R-Mich.): Upton has been a conservative target for years, but 2012 may represent his most serious challenge yet. Former state representative Jack Hoogendyk, who lost to Upton 57 percent to 43 percent in 2010, is back again. But this time, he could well have the all-important backing of the Club for Growth. Interestingly, former Pennsylvania senator Rick Santorum carried Upton’s 6th district 43 percent to 37 percent over former Massachusetts governor Mitt Romney in the Michigan presidential primary. Upton endorsed Romney; Hoogendyk backed Santorum.

3. Chuck Fleischmann (R-Tenn): Fleischmann won this east Tennessee seat in 2010 when then-Rep. Zach Wamp ran unsuccessfully for governor. But now he faces a serious primary challenge from another Wamp — Weston Wamp, the 25-year-old son of the former congressman. Wamp's last name and his father's connections in Washington ensure he will be a well-known and well-funded opponent for the still-new-to-Washington Fleischmann.

2. Silvestre Reyes (D-Texas): Reyes has never faced a tough primary — until now. He’s been targeted by former El Paso City Councilman Beto O’Rourke, a young reformer, and the Campaign for Primary Accountability. The eight-term congressman has been defending himself on television, but one of his recent ads drew heat for falsely claiming his opponent wanted to legalize all drugs. Reyes may be the Democrat most likely to lose his primary.

1. Tim Murphy (R-Pa.): This is a classic establishment-versus-tea party fight. Murphy’s opponent — 27-year-old former Senate aide Evan Feinberg — has been endorsed by Sens. Rand Paul (Ky.) and Tom Coburn (Okla.). (Feinberg used to work for Paul.) Murphy’s campaign, meanwhile, is touting polling that shows him with a massive lead, but the Campaign for Primary Accountability has pledged to spend $200,000 on ads hitting Murphy. That sort of spending — plus his support from the tea party wing of the GOP — could help Feinberg close the gap quickly.

Chris Cillizza and Rachel Weiner contributed to this report.