"This is the single greatest witch hunt of a politician in American history," the President tweeted.
Mr. Trump did not detail what he believes the "illegal acts" were under the previous administration, but during the 2016 campaign, he routinely called for an investigation of former Secretary of State Hillary Clinton, both on her private email server, and the work of the Clinton Foundation.
"If I win, I am going to instruct my Attorney General to get a special prosecutor to look into your situation," Mr. Trump said in the second debate with Clinton last October.
"Because there has never been so many lies, so much deception. There has never been anything like it. And we’re going to have a special prosecutor," he added.
While the President made that promise - which often brought on chants of "Lock her up!" at his campaign rallies - there is no indication that he has taken any steps to follow through on that, since becoming President almost four months ago.
In an official White House statement issued last night, Mr. Trump said nothing about the Obama Administration, instead predicting that an investigation will find "no collusion between my campaign and any foreign entity."
On Capitol Hill, most Republicans had little to say about the appointment of Mueller, a former FBI Director who held that post between 2001 and 2013 - but those who did had nothing but good things to say about the former FBI chief.
"Robert Mueller is a man of utmost integrity and character," said Rep. Michael McCaul (R-TX).
"Robert Mueller’s fair, even-handed approach will ensure he follows evidence, brings forth facts & illuminates truth," said Rep. Randy Hultgren (R-IL).
About the Author