Here's where Republicans and Democrats have stopped supporting their candidates

Party committees and a pair of leading super PACs have canceled significant fall TV advertising reservations in key House races over the past few weeks, while other districts have come online as they've grown increasingly competitive.

With just over a month to go in the midterm elections, Republicans and Democrats have hit the point where they're pulling support from certain races in order to strategically focus elsewhere, and the consequences of those moves are reverberating across the House landscape.

Taken together, the moves show Republicans on defense, cutting overlap and shifting resources to critical races, while Democrats are looking to expand the map, the latest evidence of Democratic momentum.

Among the notable moves: in MI-08 and KS-03, which are Toss Ups in CNN's Key Race Ratings and in CO-06 and PA-17, which are Lean Democratic, Republicans have canceled ads in an attempt to triage races trending in the wrong direction.

Meanwhile in VA-10, which is held by Republican Barbara Comstock and rated Lean Democratic by CNN, and CO-06, Democrats have canceled ads for the opposite reason -- confidence in their prospects.

Both party groups have also started pouring resources into new districts for the first time.

Republicans have expanded ad reservations in IA-03, WI-01 and NM-02, while Democrats have entered the fray in PA-16, NC-02, WV-03 and NY-24.

One thing all these seats have in common (excluding Pennsylvania, which was redistricted earlier this year): they were won by Republicans in 2016. And all are on CNN's list of key House races.

Ratings for races where ads cancelled:

MI-08: Toss-up
KS-03: Toss-up
CO-06: Lean Democrat
PA-17: Lean Democrat
VA-10: Lean Democrat

Ratings for races where ads added:

IA-03: Toss-up
WV-03: Toss-up
WI-01: Lean Republican
NM-02: Lean Republican
NC-02: Lean Republican
NY-24: Likely Republican
PA-16: Likely Republican

On the Republican side, the Congressional Leadership Fund, a leading super PAC aligned with outgoing House Speaker Paul Ryan, canceled ads in MI-08 (worth $2.1 million) and CO-06 (worth $1 million). The NRCC, meanwhile, canceled ads in KS-03 and PA-17.

In CO-06, GOP Rep. Mike Coffman is facing a robust challenge from Democrat Jason Crow, an army veteran. A mid-September poll from the New York Times and Siena College showed Crow leading Coffman by 11 points, 51% to 40%. Crow also doubled Coffman's fundraising in the second quarter of 2018.

The move by CLF leaves Democrats with a roughly two-to-one advantage on TV advertising in the district, according to data from Kantar Media/CMAG. But the NRCC remains committed to the district for now, releasing a new ad in the race on Wednesday.

The MI-08 race -- between Rep. Mike Bishop and Democratic challenger Elissa Slotkin in the Detroit suburbs -- is another key Democratic pick-up opportunity.

The race hasn't seen much public polling, but it's the eleventh most expensive race by TV advertising, according to data from Kantar Media/CMAG. Even as CLF pulls out of the race, the NRCC is still set to spend $3.7 million in the district (including future ad reservations), and Republicans are still outspending Democrats here by about a 3-to-2 margin.

Courtney Alexander, CLF's communications director, said that "CLF will continue to run strong field operations in these districts and will continue to conduct polling and evaluate races across the country as we do everything we can to protect the Republican Majority."

And even as they canceled ads in MI-08 and CO-06, CLF announced they were expanding their ads in a trio of other competitive House races -- $200,000 to IA-03, $1 million to WI-01, and $225,000 to NM-02. The group also added $5 million to broadcast ad reservations in the Los Angeles market, touching on a slew of key California races.

In KS-03, meanwhile, Democrat Sharice Davids is mounting a strong campaign against incumbent Rep. Kevin Yoder. A New York Times/Siena poll in late September had Davids up 8 points on Yoder, 51% to 43%, and seemingly emboldened, the DCCC is spending $1.2 million on TV ads there (including future reservations), even as the NRCC cancels its ads.

National Republicans have grown glum about Yoder's chances in Kansas' 3rd Congressional District and the fact that the planned spending has been canceled is part of a broader issue Republicans have faced in 2018: With so many districts on the line and a finite amount of money to spend, the NRCC and other groups are forced to cut off incumbents weeks before Election Day.

Still, the Congressional Leadership Fund is committed to being on air in the final weeks of the race in Kansas, according to an aide.

The PA-17 race is a unique case, the rare instance of an incumbent vs. incumbent contest in a redrawn district. Rep. Keith Rothfus, who previously represented the 12th district, is facing Rep. Conor Lamb, who won a special election to the 18th district seat earlier this year, as a result of statewide redistricting. A Monmouth poll in late July showed Lamb with a 13 point lead, and Democrats are outspending Republicans nearly 2-to-1 over the final month of the race.

The NRCC declined to comment on the moves.

On the Democratic side, House Majority PAC, CLF's Democratic counterpart, made significant cuts to ad reservations in VA-10 and CO-06, canceling just under $1 million in ads in each race. Here, though, Democrats decided to pull out because they felt good about their prospects. It's a "sign of confidence," according to a Dem aide.

In Virginia's tenth district -- among the 5 most expensive House races by TV spending before the parties began cancelling ads, according to Kantar Media/CMAG data -- Rep. Barbara Comstock is facing a stiff challenge from Democrat State Sen. Jennifer Wexton. Wexton has maintained a mid-to-high single digit lead over Comstock in polling of the race since June, and the DCCC and NRCC are still spending over $7.8 million on TV in the race combined.

HMP also announced on Wednedsay that it was launching a $1 million ad campaign in the NY-24 race between GOP Rep. John Katko and his challenger, Dana Balter -- the race hasn't been seen as particularly competitive, but HMP's new campaign represents a significant expansion of Democratic activity in the contest.

The DCCC, meanwhile, added to their list of targets, airing their first TV ads in PA-16, NC-02, and WV-03. In all three districts, which lean conservative, the buys are less than $1 million, but represent a meaningful expansion of the battlefield when Democrats need to pick up 23 seats to win the US House majority.

CNN's Dan Merica contributed to this report.

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