Outside spending can have its biggest impact in smaller races. And in a number of contests for congressional seats where there was a significant money advantage for one side, independent expenditures seemed to help push the needle.

Here are four members of the 113th Congress whose chances of winning increased after receiving a significant boost from outside nonprofits and super PACs attacking their opponents or praising them. 

At some point in the fall, for each of these winners, the non-partisan Cook Political Report shifted its ratings to these candidates' favor. In each case, the shift came after the winning candidate got a significant boost in outside money. 

NV-3: Rep. Joe Heck, R

Overall, the independent groups may have played about even, but groups backing freshman Joe Heck pushed on the gas in October, dropping about $660,000 more than the Democratic groups. That’s when Heck made a surge over John Oceguera. The race was considered too close to call until Nov. 1, when the Cook forecast said it leaned towards Heck.

Here’s who helped the freshman stay in office:

American Action Network: The dark money nonprofit that calls its ideology center-right entered the race in the final week of October with  a $740,000 TV buy aganst Oceguera.

National Republican Campaign Committee: The group was on the air from mid-September through late October with ads against Oceguera – until it let American Action Network carry the final bucket of water. Overall, the NRCC spent more than $1.6 million on the race.

KY-6: Andy Barr, R

GOP outside groups had a three-to-one advantage over Democratic groups in this Bluegrass country district, and they ended up triumphant. Incumbent Blue Dog Democrat Ben Chandler could not make up for it with his own campaign cash, as he only barely outraised Barr, a Lexington attorney who ran unsuccessfully for the seat two years ago. Earlier in the year, Cook rated this a "lean Democrat" race but shifted it to a toss up by mid-October.

Here’s who helped Barr:

NRCC: Starting in August, the group made consistent buys opposing Chandler and ended up spending over $ 1 million. Meanwhile, the DCCC only entered the race in October and spent about $300,000.

Americans For Tax Reform: The nonprofit powerhouse is headed by Grover Norquist, famous — and in some quarters infamous — for the pledge he requires against raising taxes. It may have helped solidify Barr’s advantage. It bought a big TV spot in mid-October.

WI–8: Rep. Reid Ribble, R 

Freshman Ribble beat businessman Jamie Wall by a wide margin in a race that was supposed to be closer. But by mid-October, Cook moved Ribble into the "likely" winners column. Though the total amount of outside money was low compared to other competitive House races, GOP independent groups – led by the NRCC — had a four to one advantage. Wall got very little outside help.

Here's who backed Ribble:

NRCC: The group made consistent TV ad buys for Ribble in September and early October. There was no other TV ads in the race this fall but other groups, such as FreedomWorks, produced online ads opposing Wall.

NH-2: Ann McLane Kuster, D

Attorney Ann Kuster benefited from $3.2 million in Democratic outside money, compared to $1.8 million for Charlie Bass, one of the only remaining congressional Republicans from New England. The race was still a tossup in September but moved towards Kuster’s favor by early October, according to Cook’s ratings. The main players were the DCCC and the NRCC – but while the GOP party committee nearly matched its counterpart's spending, Kuster got help from other groups while Bass did not.

Here’s who helped Kuster:

The Democratic Congressional Campaign Committee: The party spent nearly $2 million on a series of ads claiming Bass wants to “essentially end” Medicare. The first of three ads went up in late September. Then a final $750,000 purchase dropped during the final week before the election the with an ad saying that Bass would have “ended the promise of Medicare.”

House Majority PAC: The group made a $400,000-plus ad buy in late September, also linking Bass to the controversial Medicare reform plan proposed by Rep. Paul Ryan, R-Wis., this year's GOP vice presidential nominee.