Bill Kristol (L) and President Donald Trump Getty Images

One of President Donald Trump's most vocal opponents is creating a political war machine to take on the commander in chief when it comes time for him to run for re-election in 2020. Bill Kristol, who served in the administrations of Republican presidents Ronald Reagan and George W. Bush, says his nonprofit organization Defending Democracy Together is seeking a GOP candidate to run against Trump in 2020. "We are thinking of and doing preliminary work to prepare for a primary run against Trump," Kristol said in an interview on Thursday. "People aren't going to say they will run against Trump unless they have the infrastructure but I've been trying to persuade people that it may not be that difficult," he added. Kristol, a director of Defending Democracy Together, explained that he's organized a coalition of activists who are polling voters and meeting with focus groups in swing states such as New Hampshire.

He's also been engaging with three potential Republican primary candidates who have been outspoken critics of the president: Sen. Ben Sasse of Nebraska, outgoing Sen. Jeff Flake of Arizona and Ohio Gov. John Kasich. Other possible nominees on Kristol's radar are Massachusetts Gov. Charlie Baker and Maryland Gov. Larry Hogan. Sasse, Flake and Kasich have shown interest, according to Kristol, but they've also echoed what they've said publicly which is that they are focused on serving their constituency and helping Republicans maintain their majorities in Congress throughout the congressional midterm elections. Kristol is also readying a super PAC for 2020 GOP candidates which would allow his team to raise unlimited amounts of cash and push out campaign advertisements backing a particular nominee. A spokesman for Sasse did not deny the two have spoken about a prospective 2020 run. "I'm not going to comment on what other people decide to bring up in their personal conversations with the senator, but — like we've said for a long time — Ben doesn't pay any attention to the 2020 Washington rumor mill," James Wegman, a spokesman for Sasse, said. Representatives for Flake and Kasich did not respond to multiple requests for comment. Meanwhile, Kristol and his allies, including former New Hampshire GOP chairwoman Jennifer Horn, have been gauging voters on their receptiveness to a primary challenge against Trump. "I think it's likely that Donald Trump will have a primary and that it is entirely his own doing," Horn said in an interview. "His behavior as president is damaging and demeaning to the presidency. I believe it has damaged our nation."

While their internal polls continue to reflect Trump's growing popularity within the Republican Party, there are many, particularly in New Hampshire, who say it's time for someone to step up to battle Trump. Kristol pointed to the latest public poll as an example of what they're seeing in the Granite State. The poll by the New Hampshire Journal found that 4 in 10 Republicans surveyed believe a GOP challenger to Trump "would be a good thing," while 56 percent of overall respondents supported the idea. Trump lost the state by under a point to Hillary Clinton during the 2016 presidential election. New Hampshire has historically been a testing ground for those looking to run for president from both sides of the aisle. It's considered a diverse state in terms of voter political views and holds the first presidential primary in the nation. Kasich visited the state in April and is returning there in November for a speaking engagement at the Nackey S. Loeb School of Communications. Flake made his way to New Hampshire in March, while Sasse has yet to hold an event in the state. Still, a separate senior state political operative, who spoke on the condition of anonymity, told CNBC that state GOP officials have said they're hoping to see Sasse come to their "Politics & Eggs" breakfast speaking event, which historically has been where candidates go to tout their agenda in the early stages of a candidacy. An invite has not been sent to Sasse yet, this source added. Trump attended the breakfast after the GOP debate in November 2015 in the midst of a bomb-throwing presidential election. Trump's speaking engagement reportedly drew up to 600 people.

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