Republicans believe it's a "witch hunt"

"The base is solidified, but that doesn't get you more than that," said Lee Miringoff, director of the Marist Institute for Public Opinion, which conducted the poll.

The polarized views on the special counsel persist amid a recent flurry of developments in the Russia probe, following relative quiet around the midterms. In federal courts on Friday, Mueller's team is expected to detail how former Trump campaign chairman Paul Manafort allegedly breached his plea deal and to provide sentencing recommendations for ex-Trump lawyer Michael Cohen, who admitted last week to lying to Congress.

In the poll, for the first time, more Americans said they view Mueller more negatively than positively, 29 percent favorable, 33 percent unfavorable. That's a net 7-point decline from the summer, when Mueller was 33 percent positive and 30 percent negative.

Mueller's decline is fueled by Republicans — 58 percent have an unfavorable view of him in the most recent polling, up from 46 percent in July. (Just 8 percent have a favorable opinion of him, down from 15 percent in July.)

(The slight bump in March 2018 in the chart above followed several developments in late February in the Mueller investigation timeline. They included financial charges against Manafort and a guilty plea from Manafort's former business partner Rick Gates.)

The overall movement on views of Mueller, however, is within the poll's margin of error, and — perhaps more remarkably — the former FBI director remains not very well known or defined. Despite withering attacks from Trump, the plurality of Americans (39 percent) continue to say they are unsure of their feelings toward Mueller or have never heard of him.

That's very different from views of Kenneth Starr, the special prosecutor who investigated former President Bill Clinton in the 1990s. Back then, almost 6 in 10 Americans (56 percent) had an unfavorable view of Starr, according to a Time/CNN Poll in September 1998. Only 13 percent said they were either not familiar or not sure.

"Trump does very well when he has a target that he can put characteristics on, that he can make it clear in terms of why they should like or dislike someone," Miringoff said. "When it comes to Mueller, that's not occurring. [Mueller is] largely unknown, but the investigation is known. ... That stymies the president and his handlers in terms of how they like to deal with an opponent."

The investigation is also insulated to a degree — 67 percent said they think Mueller should be allowed to finish, including 51 percent of Republicans and 70 percent of independents; and three-quarters (76 percent) believe that Mueller's final report should be made public in its entirety. That includes 68 percent of Republicans.

"The issue of transparency is big," Miringoff said. "The only thing that's stuck with the base is the notion of the 'witch hunt,' but people are supportive of letting the investigation go to its conclusion. ... And they want to see the results. If you don't get to see the sausage at the end, people are not going to be very happy about that."

Trump's approval rating in the poll remains essentially unchanged at 42 percent, and fewer — just 36 percent — think the country is headed in the right direction.