Washington (CNN) - Mark Pryor's one of the most vulnerable Democratic senators up for re-election this year. Up until now he's been spared the wrath of Americans for Prosperity, the conservative group that's spent millions this election cycle attacking Democrats over the new health care law, better known as Obamacare.
But that ends Thursday, when AFP says it will go up with ads highlighting the two-term Senator from Arkansas' support for the Affordable Care Act, which is President Barack Obama's signature domestic achievement.
Pryor faces a challenging re-election this year against Rep. Tom Cotton, the likely Republican Senate nominee, in a state that voted overwhelming against Obama in 2008 and 2012.
AFP, backed by the deep pockets of the billionaire industrialist Koch brothers, says it's already spent more than $25 million since October to criticize Democratic Sens. Mary Landrieu of Louisiana, Kay Hagan of North Carolina, Mark Begich of Alaska, and Jeanne Shaheen of New Hampshire, as well as Democratic Reps. Bruce Braley of Iowa and Gary Peters of Michigan, who are both running to keep open Senate seats in party hands, and around a dozen other House Democrats, for their support of Obamacare.
Earlier Tuesday, AFP announced it was expanding its spending against Hagan.
"It's no surprise the Koch Brothers are spending a fraction of their fortune to buy themselves a U.S. Senate that works for them and against the middle class," said Democratic Senatorial Campaign Committee press secretary Justin Baransky, in response to the new push by AFP.
The Washington Post first reported the AFP ad buy.
CNN Political Editor Paul Steinhauser contributed to this report