Donald Trump’s latest tweets lashing out at Special Counsel Mueller and his team portends big trouble for Trump, for Republicans and for the country the rest of the year.  If Trump is innocent he should want the investigation to be complete and he should cooperate fully.  But he obviously doesn’t and he isn’t.
He may not have the power to directly fire Mueller but has the power, in effect, to make it happen. He can fire anyone in Department of Justice who refuses to stop Mueller and install people who will do the deed. This is what President Nixon did in desperate effort to stop Watergate investigation.  Any of that, however, would result in a Constitutional crisis, in that some (many?) Republican Congress members and Senators would join Democrats in directly challenging such a move.
You may  want to note that Mr. Trump blasts Mueller’s team as  stacked with “Democrats.” But Mueller is and long has been a Republican. When he became FBI Director some years ago he was appointed by Republican Pres George W. Bush.
Also, note that Pres Trump’s latest angry outburst directly follows news that Mueller is digging deep into Trump business connections with Russians.  Trump, despite promises, has never released his tax returns and has always been especially sensitive to anyone trying to get an answer about his overseas business dealings.
My own speculation—clearly labeled as such—is that Trump may be especially worried that Mueller is investigating possible money-laundering by Trump businesses for Russians and others.
Bottom line to latest developments is: The truth will come out eventually, but there will be a long, ugly road before we get to it.