Story highlights Rep. Adam Kinzinger would have run "literally to save the union," according to a source

Ballot deadlines for independent candidates have either passed or soon will

(CNN) Bill Kristol and other Never-Trump Republicans have done extensive polling and talked to potential candidates and financial backers about how to stop Donald Trump, according to sources familiar with those efforts.

They understand that getting on the ballot at this late stage will be a challenge but insist they can overcome that with the right candidate and resources.

Their continued work to halt Trump, the presumptive Republican nominee, comes as Rep. Adam Kinzinger, a prominent GOP congressman from Illinois and Iraq War veteran, has decided to pass on an independent bid.

Adam Kinzinger, 38 and in his third House term, would have considered mounting an independent bid had the barriers to an independent run not been so daunting. His pondering of the matter reflects ongoing efforts by GOP forces opposed to Trump and Hillary Clinton, the likely Democratic standard-bearer. They have searched feverishly in recent weeks for a candidate of stature to make an independent conservative White House bid. But like Kinzinger, no potential candidate has yet bitten.

Kinzinger, previously a prominent surrogate for former Florida Gov. Jeb Bush, would have undertaken a third party run "literally to save the union," according to a source familiar with his thinking, because both Clinton and Trump scare him. But Kinzinger does not think the infrastructure exists to get on the ballot in a number of states at this relatively late date.

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