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 RubioHillicon Valley: Senators urge Trump to bar Huawei products from electric grid | Ex-security officials condemn Trump emergency declaration | New malicious cyber tool found | Facebook faces questions on treatment of moderators Key senators say administration should ban Huawei tech in US electric grid Trump unleashing digital juggernaut ahead of 2020 MORE (R-Fla.), Ted Cruz Rafael (Ted) Edward CruzTrump unleashing digital juggernaut ahead of 2020 Inviting Kim Jong Un to Washington Trump endorses Cornyn for reelection as O'Rourke mulls challenge MORE (R-Texas) and Rand Paul Randal (Rand) Howard PaulThe Hill's Morning Report — Emergency declaration to test GOP loyalty to Trump The Hill's 12:30 Report: Trump escalates fight with NY Times The 10 GOP senators who may break with Trump on emergency 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 LeePush to end U.S. support for Saudi war hits Senate setback The Hill's Morning Report — Emergency declaration to test GOP loyalty to Trump The Hill's 12:30 Report: Trump escalates fight with NY Times 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 McConnellHouse to push back at Trump on border Democrats block abortion bill in Senate Overnight Energy: Climate protesters storm McConnell’s office | Center-right group says Green New Deal could cost trillion | Dire warnings from new climate studies 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 FlakeBrexit and exit: A transatlantic comparison Poll: 33% of Kentucky voters approve of McConnell Trump suggests Heller lost reelection bid because he was 'hostile' during 2016 presidential campaign MORE (Ariz.), Thad Cochran William (Thad) Thad CochranTop 5 races to watch in 2019 Bottom Line Races Dems narrowly lost show party needs to return to Howard Dean’s 50 state strategy MORE (Miss.), Lindsey Graham Lindsey Olin GrahamHouse to push back at Trump on border Trump pressures GOP senators ahead of emergency declaration vote: 'Be strong and smart' This week: Congress, Trump set for showdown on emergency declaration MORE (S.C.), Johnny Isakson John (Johnny) Hardy IsaksonOn The Money: Lawmakers wait for Trump verdict on border deal | Trump touts deal as offering B for security | McConnell presses Trump to sign off | National debt tops T | Watchdog details IRS shutdown woes Trump criticizes border wall deal: 'Can't say I'm happy' GOP senators offer praise for Klobuchar: 'She’s the whole package' MORE (Ga.), Richard Burr Richard Mauze BurrHillicon Valley: Senators urge Trump to bar Huawei products from electric grid | Ex-security officials condemn Trump emergency declaration | New malicious cyber tool found | Facebook faces questions on treatment of moderators Key senators say administration should ban Huawei tech in US electric grid Five tantalizing questions about Mueller’s investigation MORE (N.C.) and Lamar Alexander Andrew (Lamar) Lamar AlexanderPence meeting with Senate GOP ahead of vote to block emergency declaration Addressing repair backlog at national parks can give Congress a big win The Hill's Morning Report — Emergency declaration to test GOP loyalty to Trump 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 McCainGOP lobbyists worry Trump lags in K Street fundraising Mark Kelly kicks off Senate bid: ‘A mission to lift up hardworking Arizonans’ Gabbard hits back at Meghan McCain after fight over Assad 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 ReidBottom Line Brennan fires back at 'selfish' Trump over Harry Reid criticism Trump rips Harry Reid for 'failed career' after ex-Dem leader slams him in interview 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.