*updated at 1:00pm
Washington (CNN) - They are two of the hottest battlegrounds in the 2014 fight for control of the U.S. Senate.
And Wednesday a leading pro-Democratic group said it's going up with TV commercials in North Carolina and Arkansas that target Republicans challenging incumbent Democratic Sens. Kay Hagan and Mark Pryor, who face tough re-elections this year.
Patriot Majority USA says it's launching the ads "to combat the waves of money flooding in from the Koch Brothers and other right wing outside groups."
The conservative group Americans for Prosperity, backed by the deep pockets of the billionaire industrialist brothers, David and Charles Koch, has spent more than $8 million since October to run spots in North Carolina, and earlier this month AFP started going up with commercials against Pryor in Arkansas.
The new ad in North Carolina, titled "Bad Company," highlights how Republican state House Speaker "Thom Tillis sides with health insurance companies," said a statement from Patriot Majority. Tillis is considered to be the leading candidate among the GOP hopefuls hoping to unseat Hagan in North Carolina.
The spot in Arkansas highlights how Rep. Tom Cotton was the only lawmaker in the state's congressional delegation to vote against the Farm Bill. Cotton, a freshman congressman who's considered a rising star in the GOP, is the party's likely Senate nominee to challenge Pryor.
Patriot Majority USA tells CNN they plan to spend around $500,000 in North Carolina and some $300,000 in Arkansas to run the commercials over the next three weeks.
AFP, which has spent more than $25 million since last autumn to criticize Democratic senators up for re-election and Democratic Senate candidates, announced Wednesday that it was going up with a new spot in Louisiana that attacks Democratic Sen. Mary Landrieu for her continued support for the health care law. AFP says the ad will run for the next three weeks and calls the buy "significant."
(note: an earlier version of this story incorrectly stated that the new AFP ad was targeting Democratic Sen. Jeanne Shaheen of New Hampshire)
The Democrats currently have 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 for grabs in November's midterm elections. And half of the seats the Democrats are defending are located in red or purple states.