The FBI raids conducted against President Donald Trump's longtime personal attorney have sent shockwaves through Republican leadership ranks as Democrats look to flip both houses of Congress in November's midterm elections.

Political operatives close to congressional GOP leadership are shaken after the FBI raided the office and residence of the lawyer, Michael Cohen, on Monday. These external advisors spoke to CNBC on the condition of anonymity as they are heavily involved with the elections.

Republican political advisors think the issue could push voters away from the GOP agenda and give Democrats a big boost in the midterms.

"Among strategists, concern is at an all-caps level," said one Republican strategist who is close to Senate leaders.

The reason for the fear, this strategist said, comes from uncertainty about how the president may react, including possibly firing special counsel Robert Mueller, who reportedly made the referral leading to the Cohen raid, and Deputy Attorney General Rod Rosenstein, who reportedly signed off on the raids.

Trump on Monday called the investigation "an attack on our nation."

There also appears to be a concern, the strategist said, about potential charges being brought against Cohen, who is one of Trump's closest confidants. Cohen is known to have extensive knowledge of the inner workings of the Trump Organization.

"Most operatives are monitoring the situation closely with the hope for some sign of relief," the strategist said. "I don't think anybody knows what to expect. To us, the unknown is the scariest part."

Another strategist close to GOP leaders shared similar concerns — and called for Republicans to distance themselves from the situation before it hurts them in November.

"I imagine people are watching closely while trying to stay as far away as possible," the GOP operative told CNBC.