A Special Investigator has to be approved by Congress, so that won't happen. What will happen is the Intelligence Committee will investigate. They're already gearing up for it. Congress has subpoena authority, but the President also has many, many exemptions, due to the separation of powers. I'm not sure if he can ever be compelled to produce his tax returns, although if it got to that point, the IRS has them, so it is not like he has them in a secret location!!!
I'd be surprised if Putin didn't already have a copy!
The key to all of this is the tipping point at which Republicans in Congress believe it is hurting them in their own districts. Only then will we see a sea change.
That said, there are Congressional elections next Autumn, so some of the Republicans need to get out ahead of this. However, they have been so successful at gerrymandering that many, many Republicans may NEVER have to worry about losing their seats!
Democracy, my arse! AustinBlue should know, he lives in a very very liberal/Democratic city in Texas that NEVER goes to Democrats because the city has been divvied up into small pieces attached to the Republican rural areas surrounding the city!