Americans for Prosperity is set to unveil on Thursday a three-week air attack against Arkansas Sen. Mark Pryor, one of the most vulnerable Senate Democrats seeking reelection this fall, who had, until now, been spared the intense ad blitz the group has unleashed across the country.

The new television ad will spotlight Pryor's support for Obamacare, which the conservative advocacy group has been using to pummel Democrats.

The latest AFP spending against Pryor means that five incumbent Democrats in the Senate facing some of the steepest odds in November are in the crosshairs of the tax-exempt organization, which serves as the main political vehicle of the billionaires Charles and David Koch and their network of allied donors.

On Monday, AFP announced it is expanding its spending against North Carolina Sen. Kay Hagan, plowing another $1.4 million into an ongoing campaign noting her support for President Obama’s signature health-care legislation, as Politico first reported.

Justin Barasky, a spokesman for the Democratic Senatorial Campaign Committee, said in a statement, "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."

AFP is also going after Democratic candidates running for open seats in Michigan and Iowa, investing money that could help Republicans expand the Senate battlefield. And the group is amping up the pressure on House Democrats, unleashing a new ad this week against Rep. Nick Rahall of West Virginia.

With the new buys, the nonprofit group has already spent more than $27 million since the fall – putting it on pace to far outstrip the $38.5 million it spent during the 2010 midterms.

More Senate and House targets are still to come, said AFP president Tim Phillips, adding: “We’re definitely not finished.”