The Constitution provides impeachment as the vehicle to remove someone from office. It it says nothing about providing immunity for anybody from any prosecution. If you can find an article or section that says otherwise please point it out to me.
That’s the counter argument. And a pretty good one, but at the same time you can see the problems that would occur if you allowed the President to be indicted without first being impeached. The Democrats have a long history of using criminal charges for political purposes. A lone prosecutor could bring the entire government down even if the Congress refused to impeach.