Washington (CNN) Six months ago, Republican voters were almost evenly split on whether special counsel Robert Mueller was conducting a fair investigation into potential links between President Donald Trump's campaign and the Russian government.

Not anymore.

For the first time, a majority of Republican voters, 54%, now believe Mueller is not conducting a fair investigation, up 10 percentage points over the last six months, according to a new Quinnipiac University poll released this week.

Only 3 in 10 Republican voters, 30%, would say Mueller is conducting a fair probe into potential collusion, down 11 points over the last six months. Overall, however, a majority of American voters, 52%, believe Mueller's probe is being conducted fairly.

The survey was conducted almost entirely before federal investigators on Monday raided the office and hotel room of the President's lawyer, Michael Cohen. That day, Trump called the raid a "disgraceful situation" and "an attack on our country," saying the investigation had reached a "whole new level of unfairness."

Read More