Democrats hold up their win in a special election last May in upstate New York when Democrat Kathy Hochul linked Ryan's proposal to her GOP opponent Jane Corwin and took over what had been a reliably red seat.

But one top GOP strategist told CNN that that upstate New York special election "was the best thing that ever happened to us. It prepared us, it opened us up, and it gave us a year to explain our plan."

Other Republican aides say the national congressional campaign committee regrouped last fall and developed a response that they used in a special election in Nevada.

Republicans argued it was the Democrats who were gutting Medicare -- by using more than $700 million in reductions for payments to physicians to pay for Obamacare. The GOP candidate Mark Amodei won that election and the strategy has been replicated by candidates ever since.

While Republicans admit Democrats have traditionally had an advantage when it comes to Medicare they believe House GOP candidates have been able to neutralize the issue by not letting charges go unanswered and by tying the debate to the economy.

"They've put all their eggs in a message basket that didn't work," a senior official from the NRCC told CNN.

With the bulk of this cycle's competitive races concentrated in districts represented by the more moderate members of each party, the outcome of this election will mean an even more polarized House in 2013.

The GOP conference will include even more conservatives and the likely loss of more moderate Democrats, whose ranks were already decimated in 2010, will tilt the Democratic caucus further to the left.

A recent study by the Cook Political Report of all congressional districts found that the number of swing districts in the United States went from 164 to 99 in the last 14 years. That decline will undoubtedly have an impact on the ideological divide between the two parties in the House not just this year, but in future elections.

"There's a remarkable reduction in the number of members who have an incentive to compromise," Wasserman told CNN.

Key House race snapshots

Compiled by Adam Levy and Robert Yoon, CNN Political Research

Arizona 1: Jonathan Paton (R) vs. Former Rep. Ann Kirkpatrick (D)

Open Republican-held seat

This redrawn district covers most of northern and eastern Arizona. The Democratic nominee is Ann Kirkpatrick, who was elected in 2008 and was swept out in the Republican wave two years later. The Republican nominee is Jonathan Paton, a former state senator. Kirkpatrick has a sizeable fundraising advantage over Paton, but national Republicans have invested heavily in this race to help close the gap in TV ads. This seat is a top priority for both parties.

Arizona 2: Rep. Ron Barber (D) vs. Martha McSally (R)

This is the district that Democrat Gabrielle Giffords would have run in had she sought a fourth term. Giffords was shot and wounded in January 2011 in Arizona and continues to recover. Her district director Ron Barber was also injured but won a special election to fill her seat when she resigned from the House last January. His opponent is Republican Martha McSally, a retired Air Force colonel and combat pilot. Barber had a significant financial advantage at the start of October though McSally has remained competitive on the airwaves. Still, Barber is expected to win.

Arizona 9: Kyrsten Sinema (D) vs. Vernon Parker (R)

New district

The battle for this new district pits Democrat Kyrsten Sinema, a former state senator, against Republican Vernon Parker, the former mayor of Paradise Valley. Both parties have invested heavily in the race, though a Democratic super PAC has tipped the TV ad war balance of power slightly in the Democrat's favor.

California 30: Rep. Brad Sherman (D) vs. Rep. Howard Berman (D)

Merged district

One of the nastiest House races of 2012 is between two incumbent Democrats due to redistricting and the state's new primary system, where the top two finishers advanced to the general election. Rep. Brad Sherman has at least one advantage over Rep. Howard Berman: He represents more of the redrawn Sherman Oaks-area district. Sherman won the primary with 42.4% of the vote compared to 32.4% for Berman. The remaining votes were split among five candidates. The bitter race hit a nasty patch in mid October when the two men shouted at each other at a public forum. The moment was caught on video and a police officer could be seen taking the stage preparing to intervene. Both have received heavyweight endorsements. Independents and Republicans could play the decisive role.

California 31: Rep. Gary Miller (R) vs. Bob Dutton (R)

Seven-term incumbent Rep. Gary Miller faces a tough challenge from a fellow Republican in a race also determined by California's new primary system. Miller's opponent is Bob Dutton, a state senator and businessman. Miller does not currently represent any of this redrawn district, while Dutton represents much of it in the state legislature. Money could play a decisive role in the race. At the start of October, Dutton had only $58,000 in the bank, compared to the $816,000 in Miller's war chest as of October 17. Miller edged Dutton in the June top-two primary, 26.7%-24.8%, with the remaining 49% divided among four Democrats. Those Democratic voters will likely decide whether Republican Miller returns to Washington for an eighth term.