The answers to this question - What reasons may Donald Trump have had for firing FBI Director James Comey? - appear to suggest that there is widespread certainty that the sacking was politically motivated. And furthermore that the political motivation was to avoid or reduce the impact of potentially serious corruption charges.
This is not the first time Trump's behaviour has set off these alarm bells. Similar accusations were levelled against his sackings of Sally Yates and Preet Bharara.
As an outside observer (I live in Europe), it seems quite astonishing and appalling that he has been able to do this without political opposition. Especially when media condemnation and certainty that he has ulterior motives seems almost universal.
Part of the job of the US constitution and legislature would seem to be to offer checks and balances against the actions of the President. Why has no-one yet used any of these powers to try and rein in or remove the President? What steps would there have to be, now, in order to begin such a process?