Republicans are wasting no time getting ready for the 2018 Senate campaign, as several outside GOP groups have raised a record amount of money for the upcoming midterm elections.

Campaign finance groups allied with Sen. Majority Leader Mitch McConnell, R-Ky., have assembled a $31.6 million war chest as of the start of this year, according to USA Today, a record for an off-cycle year. Those groups include the Senate Leadership Fund, One Nation, Crossroads GPS and American Crossroads.

"We start this cycle better prepared than we ever have been," Steven Law, the CEO of American Crossroads, told the newspaper.

The money will go toward defending eight Senate seats held by Republicans that are up for re-election this fall. While the president's party typically loses seats in midterms, Democrats face a number of strong challenges of their own, based on a CNBC analysis of past voting patterns.

Republicans will be defending just 8 of the 34 seats that are up for re-election in November, most of which are in solidly red states. Democrats, on the other hand, are defending seats in states that have a pattern of voting Republican for both Senate and presidential candidates.

In North Dakota, for example, Democratic Sen. Heidi Heitkamp will be defending a seat she won in the 2012 election cycle by roughly 1 percent of the vote. North Dakota has heavily favored Republicans in both the 2010 and 2016 Senate cycles and in the 2012 and 2016 presidential races.