Advocacy groups that have pressured congressional Republicans to demand that a new budget measure defunds Obamacare are now turning their sights to Democrats, seeking to pin the blame on them for the government shutdown.

Heritage Action for America announced Thursday that is launching a  $400,000 online ad campaign against four Senate Democrats who face tough reelection fights in 2014: Mark Begich of Alaska, Mark Pryor of Arkansas, Mary Landrieu of Louisiana and Kay Hagan of North Carolina.

“Kay Hagan promised to represent you,” says a male narrator in one of the ads, as ominous music plays over images of the Capitol at night, a darkened office and newspaper headlines about the shutdown. “Instead Kay Hagan is working for Harry Reid and Barack Obama, to protect Obamacare. Why won’t Kay Hagan stop the shutdown and defund Obamacare?”

Heritage Action, the advocacy arm of the Heritage Foundation think tank, has been a key player in rallying support for the “Defund Obamacare” movement. In a series of town hall meetings it held around the country in August, Heritage leaders such as former senator Jim DeMint chastised GOP leaders for not backing their efforts.

The Senate Conservatives Fund, another group that has aggressively gone after Republicans, also shifted its focus Thursday to Democrats. In an e-mail to supporters, executive director Matt Hoskins asked for help raising at least $100,000 to run an ad going after Democrats.

“The first phase of this campaign was to get Republicans to stand strong,” Hoskins wrote. “While 25 Senate Republicans led by Mitch McConnell (R-Ky.) betrayed us and helped the Democrats fund Obamacare, we kept House Republicans from backing down. The next phase of this campaign is to pressure Democrats.”

The ad, titled “Listen,” praises Republicans who “have voted to fund the government, without funding Obamacare."

“But Harry Reid and the Democrats in Congress are forcing a government shutdown to get their way,” a male narrator says.

The change in targets comes after Republicans have heatedly complained about the tactics of the pro-defund advocacy groups and their criticism of GOP members. But Hoskins said that Senate Conservatives Fund was not shifting its approach in response to a backlash.

“This has always been our strategy and we would have started pressuring Democrats sooner if the Republicans had not betrayed the voters earlier in this campaign,” Hoskins wrote in an e-mail to The Post. “We don't care what our critics say. We are playing to win and we will continue to take the necessary steps to achieve that goal. Now that Republicans are beginning to win the argument against Obamacare, we hope the Republican establishment will stop attacking conservatives and join the fight. It will be a real shame if they side with the Democrats and cause us to lose such an important fight.”