Should a prosecutor (or committee) that is independent of Congress be doing the investigation of Trump's Russian scandal? The American public says yes. They don't trust Congress to get to the truth in an impartial manner. After watching the initial moves of the House committee, the public thinks both parties will instead just try to justify their own ideological positions -- with the Democrats trying to impeach Trump and the Republicans trying to cover for him.
They are probably right. This is a very serious matter, and if some of the allegations are proven to be true -- it could easily involve treasonous conduct by Trump or his aides. This can't look like a witch hunt or a whitewash -- and the only way to accomplish that is for the investigation to be conducted by a neutral figure, independent of Congress and agreed to by both political parties.
The chart above shows that in every poll done on the subject, it is the wish of the general public that an independent investigation be done. The numbers shown are from the Harvard / Harris Poll, the Quinnipiac University Poll, the Public Policy Polling survey, and the Associated Press / NORC Poll.