Heritage Action for America has a message for 100 House Republicans: You want to sign freshman Rep. Mark Meadows’ letter.

The advocacy group launched a $550,000 online ad campaign Monday that targets GOP lawmakers who haven’t yet signed on to the petition being circulated by the North Carolina Republican.

The full text of the letter and final list of co-signers won’t be made public until Thursday, Meadows’ office told CQ Roll Call. But when it is sent to House Republican leaders, it will demand that they “take the steps necessary to defund Obamacare in its entirety, including on a year-end funding bill like a continuing resolution.”

Spokesman Dan Holler wouldn’t confirm whether the 133 members not included on Heritage Action’s target list are ones who have already signed Meadows’ letter, saying only that “a bunch of these folks come from conservative districts, and they have conservative constituents who aren’t having their views represented in Washington.”

Meadows’ congressional office also demurred but did tell CQ Roll Call on Tuesday morning that, since Monday, “between four and eight” lawmakers had committed to attaching their names to the effort.

On Monday, the Heritage Action list of holdouts included Speaker John A. Boehner, R-Ohio, and his top lieutenants: Majority Leader Eric Cantor of Virginia, Majority Whip Kevin McCarthy of California, Conference Chairwoman Cathy McMorris Rodgers of Washington, Conference Vice Chairwoman Lynn Jenkins of Kansas, Policy Committee Chairman James Lankford of Oklahoma and National Republican Congressional Committee Chairman Greg Walden of Oregon.

Boehner said before the August recess that “no decisions have been made on the CR” in regard to whether the leadership would make its support for funding the government past Sept. 30, when current appropriations expire, conditional on defunding the 2010 health care law simultaneously.

But when Meadows’ letter is released later this week, it could become clear whether some of Boehner’s allies not on the Heritage Action list have “gone rogue.”

The target list, for instance, does not include the names of 11 of the 21 chairmen of the House’s standing committees, including Budget Chairman Paul D. Ryan of Wisconsin, Financial Services Chairman Jeb Hensarling of Texas, Judiciary Chairman Robert W. Goodlatte of Virginia and Oversight and Government Reform Chairman Darrell Issa of California.

Also absent from Heritage’s target list is the eighth member of Boehner’s official House GOP leadership team, Conference Secretary Virginia Foxx of North Carolina.

Foxx’s spokeswoman, Ericka Perryman, told CQ Roll Call that the congresswoman had not signed on to the Meadows letter, while still remaining “absolutely committed to stopping Obamacare and working toward its full repeal.”

The Heritage Action target list also includes the names of some, but not all, of the House Republicans who have said publicly they don’t support threatening to shut down the government over defunding Obamacare.

Reps. Tom Cole of Oklahoma and Jeff Fortenberry of Nebraska expressed incredulity about the tactic earlier this month, as did Robert Pittenger of North Carolina and Frank R. Wolf of Virginia.

Cole and Fortenberry appear on Heritage Action’s list; Pittenger and Wolf do not.

NPR earlier reported that Heritage Foundation President Jim DeMint saying “we don’t know that” on the question of whether Obama would sign a defunding bill into law.

DeMint dismissed the potential for a government shutdown if Obama didn’t sign on to undoing his signature health care law.

“The risk of that is so much less than the risk to our country, if we implement Obamacare. And so, I’m not as interested in the political futures of folks who think they might lose a showdown with the president.”

DeMint told NPR that he thinks Obama thinks Republicans are afraid of a showdown. “And if they are, they need to be replaced,” he said.