In the 2014 election cycle, 16 congressional races have seen more than $1 million in outside spending, according to research at the Center for Responsive Politics.

Republican senatorial candidate Thom Tillis responds during a televised debate at WRAL television studios in Raleigh, N.C., in April. (AP Photo/Gerry Broome, Pool)

Here are the top six, and remember, the numbers listed do not include data from many 501(c)4 groups and other nonprofits that have spent massive amounts of money so far this year. Outside spending by Americans for Prosperity and Crossroads GPS, for example, do not appear in these tallies.

Sheila Krumholz, executive director at the Center for Responsive Politics, says that the number to watch come November is whether 2014 approaches 2012's record-breaking billion dollars spent by outside groups.

"We certainly expect to come close to this," Krumholz says, "which is really saying something. We always see a drop-off following a presidential cycle. If 2014 spending is more than $1 billion, that would upset the apple cart of history."

The fact that so much money has already been spent this year is extraordinary, she says. "Nothing springs to mind that's comparable."

"The stakes are so high with the Senate maybe up for grabs," Krumholz says. "I'm sure these groups can taste it."

1. North Carolina Senate: $8,945,953

Democrats are worried that the North Carolina Senate seat currently occupied by incumbent Sen. Kay Hagan is one they could lose this year, so outside groups spent wildly to try to get the weakest opponent possible, or at least force the Republican primary into a runoff. They didn't succeed in their mission, despite spending -- along with a few conservative groups -- nearly $4.2 million against Thom Tillis, speaker of the North Carolina House and Republican Senate candidate. The nearly $2.5 million spent by outside groups supporting Tillis probably helped. And $1.5 million was spent supporting Hagan; a reported $545,000 was spent against her. That total does not include the $7 million that Americans for Prosperity has already spent in ads that attack Hagan's position on the Affordable Care Act and other issues.

2. Kentucky Senate: $5,064,055

U.S. Senate candidate and Kentucky Secretary of State Alison Lundergan Grimes (D-KY) campaigns in advance of the state's Democratic primary on May 19 at Lakeview Park in Frankfort, Kentucky. A recent "Bluegrass Poll" by the Louisville Courier-Journal showed Grimes in a virtual tie with the incumbent, Senate Republican Leader Mitch McConnell (R-KY). (Photo by Win McNamee/Getty Images)

Much of the outside spending in the Kentucky Senate race wasn't focused on the primaries, which both incumbent Republican Sen. Mitch McConnell and Democratic candidate Alison Lundergan Grimes won resoundingly, but as a preview of the nasty general election battle that has already begun. More than $1.8 million was spent by Kentuckians for Strong Leadership against Grimes. The Chamber of Commerce spent another $1 million in support of McConnell. About $1.4 million was spent in support of McConnell, and another $1.1 million against him. About $17,000 was spent in support of Grimes. No reported outside spending was made for or against McConnell's opponent, Matt Bevin.

3. Arkansas Senate: $5,045,326

Arkansas Senator Mark Pryor at his campaign office on April 26 in Little Rock, Ark. Sen. Pryor is in a tight reelection campaign with Republican opponent , U.S. Rep. Tom Cotton. (Photo by Stephen B. Thornton for The Washington Post)

Because Democratic incumbent Sen. Mark Pryor and Republican candidate Rep. Tom Cotton were unopposed in their primaries, the Arkansas Senate race has basically been in general election mode since last year, which explains why outside groups have spent so much. A reported $2.6 million was spent against Cotton, and nearly $1 million was spent against Pryor. Again, those figures miss a lot, since Americans for Prosperity has already spent millions in issue ads that attack Pryor's positions on Obamacare. A reported $1.2 million was spent in support of Cotton, and $229,000 for Pryor.

This is one of the races that Republicans are most confident they can pick up, and that Democrats are most eager to defend. Outside pending is going to get even more insane.

4. Georgia Senate: $3,685,006

Georgia Republican Senate candidate, David Perdue, right, greets supporters after speaking at a primary election night party May 20 in Atlanta. (AP Photo/David Goldman)

The Republican Senate primary in Georgia saw lots of money partly because there were so many candidates who hailed from different corners of the party. The tea party bet their chips on some candidates, the Chamber of Commerce on their guy, so on and so forth.

The Madison Project spent $1 in support of Paul Broun, a sum which did not vault him into the runoff. The two candidates who had the most support from outside groups, Rep. Jack Kingston and former CEO David Perdue, did make it into the runoff. Rep. Phil Gingrey, one of the furthest-right candidates who quickly fell to the back of the polls, had $1.2 million spent against him by Ending Spending.

The Republican primary is sure to get even more expensive between now and July 22, and outside groups will spend marathon amounts once the general election begins.

5. Nebraska Senate: $3,081,587

Republican Senate hopeful Ben Sasse, center, laughs with Rep. Jeff Fortenberry, R-Neb., left, and Neb. Gov. Dave Heineman in Lincoln, Neb., on May 13 after winning his party's primary election. (AP Photo)

Even Nebraska isn't immune to outrageous early spending. The Senate Conservatives Fund and Club for Growth went all out for Ben Sasse, who won the Republican Senate primary. A total of about $1.4 million was reportedly spent in support of Sasse, and more than $1 million was spent against his two Republican opponents. Sasse has a gigantic lead in the polls over Dave Domina, the Democratic nominee, for the open Senate seat.

6. Idaho District 2: $3,068,042

We decided to throw in an extra, just to show that some House races have seen insane spending too. The Republican primary for this race just ended on Tuesday -- with far more of a whimper than all this outside money would imply. Club for Growth was an enthusiastic supporter of Bryan Smith, the tea party challenger in the race, before they suddenly weren't a few weeks before the primary election. They spent more than $500,000 in the race. Outside groups spent nearly $2.5 million in support of Simpson or against Smith. They spent more than $607,000 supporting Smith or opposing Simpson. Smith lost by 23 percent.

Idaho is a reliably red state and Simpson is an incumbent, so he should have an easy victory.