Please note the Mike Flynn did absolutely nothing wrong by calling the ambassador.
That's not the point. He apparently lied about the substance of his conversations to Pence, who repeated the lie to the American public. If transcripts of those conversations exist, as apparently they do, Flynn's position became untenable because he had become a target of potential blackmail.
Of course, the trouble for Trump is just beginning, because now the issue has been raised directly about Trump's and his campaign's ties to the Russians. If the Trump campaign, for example, assisted the Russians in hacking the DNC e-mails, the Trump Presidency is toast. And now the clamor will be irresistible to get to the bottom of those alleged ties, come what may. And it will be bi-partisan clamor.
That's why my bottom line is the GOP needs to protect Pence. It is time to start planning for the denouement.