Saying that 20% of Republicans are actually Democrats is tricky because the actual number of Republicans who vote Democrat is a fluid number depending on the number needed for the Dems to win the vote.
We have to start by acknowledging that the Democrats will always win in Congress. They always do. So, you take the number of Democrats and add to that whatever number of Republicans are needed. If the Dems need only a few votes, then only a few Republicans will support them. If the Democrats need a dozen votes, then miraculously a dozen Republicans will vote Democrat.
Most Republicans still want to pretend they are Republicans. So they only vote Democrat when they have to, in order for the Democrats to win. If the Dems can win without their vote, they will vote Republican to keep up appearances for the next election.
But ultimately, the Democrats will always win in Congress.
I don't know how many Republicans it would take to overcome this automatic bias. Certainly a one or two majority is not nearly enough. Maybe if there was only 10 Democrat Senators, maybe the Republicans would win? But I'm not even sure that would be enough. As much as the establishment GOPe hates Trump, even 10 Dem Senators with Republican help, may still be enough to win.
One thing we have learned for sure with the election of Trump, is that Congress is bullshit. Political Parties are all bullshit. It is all a pre-negotiated, carefully planned show for the idiots (you and me) who can be fooled with smoke and mirrors and kabuki theater. The Republicans have no intention of doing anything. They are just killing time until the next election when they can put the Democrats back in charge to take the pressure off themselves.