Republicans prepare to unleash attacks against Jon Tester citing his efforts to block Ronny Jackson as well as his work as chairman of the Democratic Senatorial Campaign Committee.

Sen. Jon Tester (D-MT) Getty Images

Democratic Sen. Jon Tester of Montana still doesn't know who his Republican opponent will be in the November midterm elections. But the Democratic lawmaker is already under fire from President Donald Trump and other GOP critics who blame him for scuttling Ronny Jackson's bid to become the secretary of Veterans Affairs. Now, conservative politicians and groups, including one linked to Senate Majority Leader Mitch McConnell, are considering flooding the airwaves with attack ads against the senator that carry a dual message: Tester has failed to protect veterans, and he hasn't lived up to his promise of avoiding K Street lobbyists. "Tester is the nail, and the message of what he did to Adm. Jackson is going to be the hammer because it's also representative of the way veterans in general have been treated by the VA," Troy Downing, one of the numerous Republicans vying for Tester's seat and a veteran himself, told CNBC. "And despite Tester's protestations to the contrary, he has failed them." Tester, a member of the Senate Veterans' Affairs Committee, publicized allegations against Jackson as the White House physician's confirmation hearing neared. Jackson was accused of being drunk while on duty and of handing out drugs without prescriptions, among other things. Jackson eventually withdrew his nomination, which led the president to call on Tester to resign.

It remains unclear which political action committees will seize on Tester's efforts to derail Jackson's nomination. However, the Senate Leadership Fund, or SLF, a super PAC led by allies of McConnell, R-Ky., has not ruled out crafting new attack ads against the Montana Democrat. Ads could potentially target Tester's effort against Jackson as an attack on veterans, according to sources familiar with SLF's plans. The group has already published numerous digital ads against Tester, including one in early April that went after his record on the VA, which has struggled at times to help veterans with their health care. A spokesman for the committee, Chris Pack, declined to comment about specific strategies but said the group will hold Tester accountable for what he describes as his strident opposition to the president's agenda in a state that Trump won by more than 20 points during the 2016 presidential election.

GOP strategists go after Tester's role as DSCC chairman

Republican operatives have also been circulating opposition research about Tester's time as the chairman of the Democratic Senatorial Campaign Committee, or DSCC, the fundraising arm for Democrats running for Senate seats. In it they claim Tester has violated a promise he made in 2006 to never back Washington lobbyists when he first ran for his current seat.

"You've got to have somebody who will go to Washington, D.C., and fight for you, somebody who will bring Montana values back to Washington, D.C. … I won't sell Montana down the road by cutting deals with K Street lobbyists," Tester said during a debate with then-Republican incumbent Conrad Burns. At the time, Tester was referencing Burns' connection to longtime lobbyist and convicted criminal Jack Abramoff, who was part of a federal corruption investigation that led to 21 other people being found guilty or pleading guilty. While Tester was in charge of the DSCC, the research shows the committee raised more than $5 million in bundled lobbying donations throughout the 2016 election cycle, the most in the past three cycles. "No politician has ever come to Washington and raised more money through lobbyists than Jon Tester," said one Republican strategist, who declined to be named because of his involvement in the election. "It's 180 degrees opposite to what he promised people when he ran the first time. He's been here for too long and he's exactly the type of politician Trump voters are looking to fire," he added.

Democrats remain unfazed and determined to hold onto Senate seat

Democrats, however, don't seem fazed by the Republican threat. A spokesman for the Senate Majority PAC, a committee that looks to spend millions in 2018 to elect Democrats to the Senate, shrugged off Republicans' attempt to use Tester's disruption of Jackson's nomination as a means to derail his campaign. "Jon Tester's strong record of working on a bipartisan basis to get real results for Montana veterans will be an asset for him throughout this election," a spokesman for the group told CNBC.

Other Democratic fundraising committees also seem to be unconcerned with the latest run-in with Trump and the Republican Party. Democrats argue that the reason the Republicans are pouncing on the controversy with Jackson now is because the GOP lacks a formidable alternative to Tester. "To imagine voters would react negatively because someone is making sure that veterans are being represented by someone qualified shows an incredible, an almost entire rejection of reality, on the part of national Republicans," said one Democratic strategist familiar with the DSCC's thinking. "I think they have had a series of recruitment failures for Montana's Senate seat and they don't seem to know how to take on Tester," the source added. The DSCC declined to comment.

Tester's record working with veterans

Democrats in the state also point to Tester's record of working with the Trump administration to help veterans. Throughout the current Congress, Tester has authored and Trump has signed eight separate bills pertaining to the VA, including the VA Accountability and Whistleblower Protection Act, and the Veterans Choice Program Improvement Act.

Tester has also worked on bills that have helped with cost-of-living adjustments for veterans, simplifying the disability appeals process, addressing VA workforce and hiring practices, and extending the G.I. Bill so it doesn't arbitrarily expire, among others signed by Trump. In a statement to CNBC, Tester didn't hold back in touting his work with veterans and noted he remains committed to assisting them. "It's my duty to make sure Montana veterans get what they need and have earned, and I'll never stop fighting for them as their Senator," the Democrat said.

Outside money pouring into Montana

Both sides have seen an uptick of outside money being poured into the state from a variety of groups. State auditor and former Republican state Sen. Matt Rosendale, who is vying for the GOP nomination, has the blessing of the Club for Growth PAC, the sister group of the nonprofit political network Club for Growth, which is financially supported by the conservative billionaire duo, Charles and David Koch. The PAC has given more than $100,000 to his campaign according to Federal Election Commission data. On the Democrat side, Tester, a member of the Senate Banking Committee, has the support of employees from some of the top Wall Street banks, including Goldman Sachs, who have shelled out $62,000 in total, according to the Center for Responsive Politics. Employees at Citigroup have donated $48,000 while executives at private equity behemoth Blackstone Group have contributed $43,000 to Tester's cause.