The Senate Conservatives Fund is going up with a small radio ad buy against Sen. Richard Burr (R-N.C.) -- a sign that conservative groups may target Burr for denouncing their Defund Obamacare campaign.

The SCF ad buy is about $40,000 and attacks Burr for labeling Defund Obamacare the "dumbest idea I've ever heard."

"What's dumb is for Richard Burr to vote to fund a government program like Obamacare that will do so much harm to North Carolina families," the narrator says, adding toward the end: "This could be our last chance to stop Obamacare, and it’s time for Richard Burr to start listening to us, not his friends in Washington."

Conservatives in the Senate want Republicans to pledge to vote against any budget that funds Obamacare, but members like Burr have publicly broken with that idea.

Other conservative groups, including the Tea Party Patriots, For America and Heritage Action, have also been targeting Republicans who don't support the pledge, launching online ads and going on tours to apply pressure.

Burr isn't up for reelection next year but will be in 2016.

SCF was formerly allied with Jim DeMint, who left the Senate this year to become the new head of the Heritage Foundation and Heritage Action.

Update 12:49 p.m. Wednesday: SCF is also running ads against Sens. Lindsey Graham (R-S.C.) and Lamar Alexander (R-Tenn.), who face primary challengers in 2014.