Washington (CNN) - The U.S. Chamber of Commerce says it's launching six new television commercials this week in five states.
The ad buy, which the pro-business organization describes as "massive," is the latest volley by the group, which is one of the biggest spenders among the outside groups so far this midterm election cycle
The six spots - running in five states - brings to nearly 20 the number of commercials the Chamber has put up in the 2014 cycle. The ads tend to support the more mainstream or establishment Republican candidates running in GOP Senate or House primaries against candidates backed by tea party or grassroots conservative groups.
Since the birth of the tea party movement in 2009, primary challenges from the right have produced major headlines and headaches for Republicans and hurt their chances of winning back the Senate from the Democrats in the past two election cycles. Candidates backed by the tea party movement and other grassroots conservatives effectively cost the GOP five winnable Senate elections the last two cycles in Nevada, Delaware, Colorado, Indiana and Missouri.
The GOP hopes to avoid a replay of what happened in 2010 and 2012, and outside groups, like the Chamber and American Crossroads (co-founded and steered by Karl Rove) are lending a hand.
"We will aggressively support those candidates who plan to campaign on a free enterprise and growth agenda, have the courage to govern and the ability to win," said U.S. Chamber of Commerce national political director Rob Engstrom.
Republicans need to win back six Democratic-held Senate seats to gain a majority in the upper chamber. The Democrats hold a 55-45 majority in the Senate (53 Democrats and two independents who caucus with the party), but are defending 21 of the 36 seats up in November, with half of those Democratic-held seats in red or purple states.
One of those states is North Carolina, where Republicans think they have a good shot at unseating first term Sen. Kay Hagan. One of the six commercials the Chamber's launching attacks Hagan, saying "Washington Kay Hagan. Wrong for North Carolina."
The spot supports North Carolina House Speaker Thom Tillis, the establishment candidate in next Tuesday's GOP Senate primary. The commercial describes Tillis as "a bold conservative who balanced our budget and reduced regulations. A businessman who delivered tax relief."
Tillis also has the backing of Crossroads, which says it's spent $1.6 million to support the candidate. And Tuesday Republican Gov. Pat McCrory endorsed Tillis, who is facing off against Greg Brannon, a first time candidate who's backed by many conservative and tea party organizations, and by Sens. Rand Paul of Kentucky and Mike Lee of Utah. The two other major candidates in the race are minister Mark Harris, who enjoys the support of former Arkansas Gov. Mike Huckabee, and Heather Grant.
Tillis, who is hoping to avoid a runoff, is touted by the Chamber as the only candidate capable of beating Hagan in November's general election.
The group's also going up with a new ad in Georgia, where primary day is just three weeks away. The spot backs Rep. Jack Kingston, who it highlights as a "conservative fighter."
Kingston is battling fellow Reps. Phil Gingrey and Paul Broun, who are considered the more conservative candidates, as well as former Georgia Secretary of State Karen Handel, and businessman David Perdue, who's considered the frontrunner, in the contentious primary.
The winner will most likely face off against Michelle Nunn, the all-but-certain Democratic nominee, in the race to succeed retiring GOP Sen. Saxby Chambliss.
The Chamber is also going up with a new commercial in the Senate race in Alaska, where it's backing former state attorney general Dan Sullivan, in his fight for the GOP nomination against Lt. Gov. Mead Treadwell and conservative Joe Miller, the 2010 Senate nominee. The winner will face off in November against first term Democratic Sen. Mark Begich, who faces a challenging re-election.
The Chamber's also putting up money to run spots supporting the Republican Senate challengers in Michigan and Montana, two more states where the GOP thinks it has good shots of winning back seats this November. And the group says it will also begin running a commercial in North Carolina's 7th Congressional District, where the Republicans have a strong chance of winning a Democratic-held open House seat.
The new spots are the just the latest example of the Chamber's support of establishment or mainstream Republicans over candidates backed by the conservative grassroots. The group's spent big bucks to support Senate Republican Leader Mitch McConnell, as he fights off a primary challenge in Kentucky from businessman Matt Bevin, who's backed by tea party activists and other conservative groups. And it just went up with an ad staring 2012 GOP presidential nominee Mitt Romney in support of eight-term Rep. Mike Simpson of Idaho, who faces a serious primary challenge from tea party favorite Bryan Smith.