They are the seats that always get away.

Like Captain Ahab’s everlasting search for the great white whale, parties pursue these House districts with a vengeance and without success every cycle.

On paper, these districts should be easy targets — the lowest-hanging fruit to pick off the national map. But for myriad reasons over the past decade — or decades, in some cases — the parties have failed to flip these seats, no matter how strong the recruit or how many millions of dollars are spent.

Last week, Arizona’s 8th district moved one step closer to becoming a white-whale seat for Republicans. The special election to replace ex-Rep. Gabrielle Giffords (D) marked the GOP’s best chance to pick up the Tucson-based district, and their nominee lost by 8 points. This fall, Rep.-elect Ron Barber (D-Ariz.) will seek a full term in a district that is slightly more favorable to Democrats thanks to redistricting.

These white-whale districts represent a small collection of targeted seats that survived the wave elections of the past three cycles. They are Republicans who represent districts that overwhelmingly voted for President Barack Obama — as much as 61 percent. And they are Democrats, like Rep. Jim Matheson (Utah), in districts where the president received as little as 40 percent of the vote in 2008.

And much like the Arizona seat, redistricting has changed the prognosis for many of these seats. In some cases, mapmakers put these districts out of reach; in other cases, they’re more competitive than ever.

Democrats’ White-Whale Districts

Illinois’ 10th District: Rep. Robert Dold

Freshman Rep. Robert Dold (R) has the dubious honor of representing the most Democratic district of any Member in the House Republican Conference.

Democrats have spent millions over the past few cycles to pick up this north suburban Chicago seat that Republicans have held for more than 30 years.

Republican Mark Kirk won a highly competitive open-seat race in 2000, then faced relatively easy re-elections until 2006, when the Democratic Congressional Campaign Committee didn’t even target the race despite the national wave that swept the party into power that cycle.

Marketing executive Dan Seals (D) has failed three times to win this seat. He lost in back-to-back Democratic wave years and then lost in 2010 when the seat was open because Kirk ran for Senate.

But Democrats will have their best shot yet to win the 10th district this year. The party controlled redistricting in Illinois, and the party redrew the district to make it more favorable. They even moved Dold’s wealthy Winnetka base out of the district to undercut his fundraising.

Businessman Brad Schneider (D) won a competitive Democratic primary earlier this year, much to party operatives’ relief. They viewed Schneider, a strong fundraiser with a fiscally moderate profile, as the best possible candidate to face Dold.

To show they’re serious about the seat, the DCCC already reserved more than
$3 million in fall airtime in the Chicago media market for three House races, including this one. House Republicans have yet to reserve any time there.

If House Democrats can’t win this seat in November, it will be one of their biggest blunders of the cycle.

Pennsylvania’s 6th District: Rep. Jim Gerlach

This suburban Philadelphia district should have been an easy pickup for Democrats, especially in the wave elections of 2006 and 2008. The region has trended in their favor politically over the past decade when an increasing number of independents voted for Democrats — especially suburban women.

But Rep. Jim Gerlach (R) has owned this seat ever since he drew it for himself as a state legislator during the 2002 redraw — and the next 10 years look even better for him. President Barack Obama carried his current district with 58 percent, but now he’ll seek re-election in a district that Obama won with just 53 percent.

The redrawn district stretches farther west toward the center of the state (i.e., Republicans) and away from Montgomery County (i.e., Democrats).

Truth be told, Gerlach is a pretty lucky Republican. Democrats repeatedly failed because they ended up nominating candidates who couldn’t finish the job during their best opportunity cycles.

In 2006, Gerlach faced a rematch from 2004 with Lois Murphy, an attorney and a strong fundraiser, who lost by less than 2 points. In the second, stronger Democratic wave of 2008, Democrats nominated little-known businessman Bob Roggio, and Gerlach survived by a 4-point margin.

This November, Gerlach faces physician Manan Trivedi for the second cycle in a row. Trivedi is a strong candidate, but Gerlach has probably cemented his hold on this district.

Washington’s 8th district: Rep. Dave Reichert

Democratic candidates have dipped far into their own wallets to try to defeat Rep. Dave Reichert (R) without success. Cycle after cycle, Democrats recruited strong candidates only to see Reichert squeak by with single-digit victories.

In their most recent efforts, Democrats nominated two wealthy female technology executives, Darcy Burner in 2006 and 2008 and Suzan DelBene in 2010. Now both women are seeking the Democratic nod in the nearby competitive 1st district, a marginally Democratic seat — a sign the candidates know better than to try to unseat Reichert again.

House Republicans have hailed Reichert’s survivor skills in this competitive district, especially during the Democratic waves of 2006 and 2008. House GOP leaders often refer their most vulnerable Members to Reichert, who they believe can counsel imperiled Republicans through tough races.

In all likelihood, Democrats have missed their chance to defeat the famed former King County sheriff in this exurban Seattle seat. A bipartisan redistricting commission redrew the 8th district to include safer Republican territory, and now it’s probably out of reach for Democrats — at least in the near term.

Republicans’ White-Whale Districts

Kentucky’s 6th district: Rep. Ben Chandler

Six-hundred-and-forty-seven votes. That’s how close Rep. Ben Chandler (D) came to losing to attorney Andy Barr (R) last cycle. It was the Blue Dog Democrat’s closest contest to date — and probably for the rest of his political career.

Chandler’s district got better for him after redistricting, even though it’s still conservative territory. Nonetheless, it’s clear the four-term Congressman has found a successful formula to survive re-election in the toughest of cycles.

A former statewide elected official, Chandler came to Congress following a 2004 special election. He’s won re-election with solid margins ever since except for the 2010 race.

House Republicans have indicated they want to target Chandler this year. The National Republican Congressional Committee’s first round of airtime reservations for after Labor Day included a $441,000 buy in the Lexington, Ky., media market earmarked for this race. But Republicans can pull that buy any time, and privately, they’re not too optimistic about Barr’s chances this second time around.

House Democrats aren’t worried about Chandler yet. The Democratic Congressional Campaign Committee has placed more than $46.3 million in media buys across dozens of House districts but didn’t reserve anything for this race. It’s a sign that Chandler’s colleagues believe he can stand on his own this cycle, at least for now.

Pennsylvania’s 12th district: Rep. Mark Critz

No other House Member has survived more tough races in the past two years than Rep. Mark Critz (D). And it’s only going to get harder for him.

In 2010, Critz won a competitive, high-stakes special election to succeed his boss, the late Rep. John Murtha (D). Republicans rarely attempted to unseat Murtha during his 36 years in Congress, but they spent nearly seven figures to defeat Critz in one of their most embarrassing losses of the cycle.

It got worse for the GOP. While Republicans swept seats all over the country in November 2010, Critz narrowly defeated the same opponent he faced in the special, businessman Tim Burns (R). Over the course of six months, House Republicans spent $1.5 million on two failed attempts to win this district.

Last year, Republicans redrew the district to make it more conducive to pickup. They started by forcing Critz and Rep. Jason Altmire (D) into a primary in the same district. The redrawn district’s composition favored Altmire, but Critz won by 4 points in April.

After all this, Critz still faces his hardest race yet in November. He’s running on the ballot with a president that southwestern Pennsylvanians don’t view favorably, and his redrawn 12th district is more Republican than ever.

Just days after his primary victory, the National Republican Congressional Committee made a rare early ad buy blasting Critz. House Republicans view this November as their best shot yet to pick up this seat — and they’re right.

Utah’s 2nd district: Rep. Jim Matheson

Rep. Jim Matheson (D) has been a regular face on Roll Call’s Top 10 Most Vulnerable list in his six-term Congressional career. He’s beat the odds every time, but this cycle will present his greatest challenge yet.

The Blue Dog Democrat is a strong campaigner and a political survivor. But he represents a GOP district that voted for Sen. John McCain with 58 percent in 2008.

Matheson might have finally met his match in Saratoga Springs Mayor Mia Love (R), a black Mormon and a dynamic candidate who has quickly become a top candidate in the eyes of the House GOP strategists. Republicans are more hopeful than ever that they can pick up this seat — especially with Romney on top of the ticket in this heavily Mormon state.

Matheson decided to run for re-election in the new 4th district instead of the redrawn 2nd district, which he currently represents. The political composition of the new district has a similar political profile to his current seat, but it’s mostly new territory for him. Matheson is fairly well-known in the district already because it shares a media market with his current district.

Nonetheless, this cycle marks the GOP’s best shot to finally catch Matheson.