Some Republicans who are seeking an “unrealistic” Obama-Care defunding plan are playing presidential politics, according to a conservative senator. [WATCH VIDEO]

“I think it was good for their presidential ambitions, but it’s not a realistic plan. I’m pretty sure their email lists got built [up],” Sen. Mike Johanns Michael (Mike) Owen JohannsMeet the Democratic sleeper candidate gunning for Senate in Nebraska Farmers, tax incentives can ease the pain of a smaller farm bill Lobbying World MORE (R-Neb.) told The Hill on Wednesday.

The most prominent proponents of the risky tactic, Sens. Marco Rubio Marco Antonio RubioFlorida senators pushing to keep Daylight Savings Time during pandemic Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll Intelligence chief says Congress will get some in-person election security briefings MORE (R-Fla.), Ted Cruz Rafael (Ted) Edward CruzLoeffler calls for hearing in wake of Netflix's 'Cuties' Health care in the crosshairs with new Trump Supreme Court list 'Parks and Rec' cast members hosting special reunion to raise money for Wisconsin Democrats MORE (R-Texas) and Rand Paul Randal (Rand) Howard PaulSecond GOP senator to quarantine after exposure to coronavirus GOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill MORE (R-Ky.) are considered likely presidential candidates in 2016.

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They and other conservatives such as Sen. Mike Lee Michael (Mike) Shumway LeeMcConnell shores up GOP support for coronavirus package McConnell tries to unify GOP Davis: The Hall of Shame for GOP senators who remain silent on Donald Trump MORE (R-Utah) argue that Congress should not pass legislation funding the government beyond the end of the month unless it includes language defunding or delaying the 2010 Affordable Care Act.

Many Republican lawmakers, however, fear the strategy could result in a government shutdown that would hurt their party’s brand.

Johanns said linking a possible government shutdown to the quest to defund ObamaCare has boosted the national profiles of some senators, but he warned it would not yield much benefit for other Republicans.

“It burst them to a national profile. They had a lot of talkers on radio, etc., talking about it,” he said. “My feeling is we just need to be honest with people. This has zero chance of being successful.”

Rubio responded Wednesday by framing the fight over ObamaCare as the biggest issue facing the Congress.

“There’s a disagreement about whether it’s the right tactic,” he said. “If there’s one issue that we should be willing to do everything we can, it should be this one. If there’s one issue that we should be willing to take to the limit, it’s this one. It’s that bad for the country.”

Rubio held events across Florida during the August recess to highlight what he sees as the disastrous economic consequences of the law.

Tensions are rising in the Senate Republican conference over the best tactical approach to defunding the healthcare law because some members have become the targets of pressure ads from conservative groups.

The Senate Conservatives Fund launched a $340,000 television ad buy earlier this month slamming Senate Republican Leader Mitch McConnell Addison (Mitch) Mitchell McConnellMcConnell focuses on confirming judicial nominees with COVID-19 talks stalled McConnell accuses Democrats of sowing division by 'downplaying progress' on election security Warren, Schumer introduce plan for next president to cancel ,000 in student debt MORE (Ky.) for not doing more to fight the law’s implementation.

“McConnell is the Senate Republican leader, but he refuses to lead on defunding ObamaCare,” the ad’s narrator says. “What good is a leader like that?”

The group has spent about $230,000 targeting other Republican senators with radio ads, criticizing them for not backing the threatened government shutdown. The targets include Sens. Jeff Flake Jeffrey (Jeff) Lane FlakeRepublican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style Bush endorsing Biden? Don't hold your breath MORE (Ariz.), Thad Cochran William (Thad) Thad CochranEspy wins Mississippi Senate Democratic primary Bottom Line Mike Espy announces Mississippi Senate bid MORE (Miss.), Lindsey Graham Lindsey Olin GrahamThe Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Senate Democrats' campaign arm announces seven-figure investment to boost Graham challenger Graham: Comey to testify about FBI's Russia probe, Mueller declined invitation MORE (S.C.), Johnny Isakson Johnny IsaksonBottom line New poll shows tight presidential race in Georgia Matt Lieberman faces calls to drop out of Georgia Senate race over 'racist and discriminatory' tropes in 2018 book MORE (Ga.), Richard Burr Richard Mauze BurrRep. Mark Walker says he's been contacted about Liberty University vacancy Overnight Defense: Trump rejects major cut to military health care | Senate report says Trump campaign's Russia contacts posed 'grave' threat Senate report describes closer ties between 2016 Trump campaign, Russia MORE (N.C.) and Lamar Alexander Andrew (Lamar) Lamar AlexanderTrump health officials grilled over reports of politics in COVID-19 response Now is the time to renew our focus on students and their futures CDC says asymptomatic people don't need testing, draws criticism from experts MORE (Tenn.).

“Jeff Flake won’t stand up to President Obama and join conservatives in pledging to oppose funding for the implementation of ObamaCare,” declares the narrator in one spot.

McConnell has resisted endorsing the strategy for fear it could backfire.

“In the words of [conservative columnist] Charles Krauthammer, it’s a suicide note,” Sen. John McCain John Sidney McCainThe electoral reality that the media ignores Kelly's lead widens to 10 points in Arizona Senate race: poll COVID response shows a way forward on private gun sale checks MORE (R-Ariz.) said. “Until we have 67 votes in the Senate, we’re not going to be able to defund ObamaCare,” referring to the number of votes necessary to overcome a presidential veto.

Cruz urged using the government funding resolution as leverage to defund ObamaCare when he met with conservative activists at the Family Leadership Summit in Iowa last month. He’s not letting up despite his colleagues’ discomfort.

“We’re certainly continuing to press the case on every front,” he said Wednesday.

Matt Hoskins, executive director of the Senate Conservatives Fund, defended Rubio, Cruz and Paul.

“It’s amazing that whenever an elected official takes a principled stand, the establishment questions their motives,” he said.

None of three possible 2016 White House candidates spoke in favor of the government shutdown strategy at a Senate Republican meeting Wednesday where ObamaCare was a topic, according to one lawmaker who attended.

“That wasn’t the proper forum, with McConnell being targeted by ads because of their effort,” the source said.

The effort to use a possible government shutdown as leverage has put Paul in a tricky situation. He has signed a letter advocating for the tactic but he’s also an ally of McConnell, who could face a tough challenge in his state’s primary election next year. McConnell’s challenger in that race, Matt Bevin, has ripped the minority leader on ObamaCare.

When asked about the potential political impact on McConnell, Paul said: “Everybody has their own ideas about the best strategy for how to defeat ObamaCare, so I haven’t been part of any criticism of other senators for what strategy they determine is best.”

Cruz on Tuesday slammed a proposal offered this week by House GOP leaders as ineffective because it would force the Senate to vote on defunding ObamaCare before considering an extension of government funding. The House plan would not make government funding contingent on defunding the law.

“They should not use any procedural chicanery to enable [Senate Majority Leader] Harry Reid Harry Mason ReidSenate Republicans signal openness to working with Biden Mellman: The likely voter sham Bottom line MORE [D-Nev.] to circumvent that vote,” he said.

One Republican who attended Wednesday’s meeting said many of his colleagues have yet to take a position on the House resolution, which lacks the votes to pass the lower chamber. Some GOP members want to push for delaying ObamaCare while others prefer defunding it.

There is growing concern among Senate Republicans about funding levels in the House bill, which would set spending at $986.3 billion through December, according to a Senate source. Several senators say that would exceed the $967 billion spending cap set by the 2011 Budget Control Act.