In recent days, Trump had been relatively quiet about the investigation.
The barrage comes as Trump's legal team is believed to be finalizing written answers to Mueller, concerning questions about whether the Trump campaign colluded during the 2016 campaign. Mueller has been asking to interview the President for almost a year, and Trump's team has delayed responding.
One thing Trump will not be answering questions about: firing FBI Director James Comey.
The letter is a full-throated rebuke of President Trump's risky decision to fire Attorney General Jeff Sessions and his potential to now obstruct justice.
Mueller wants to ask the President questions about whether there was any obstruction of justice during the course of the investigation, which began late in the Obama administration.
As others have already pointed out, Mueller also wanted Trump to be among the last witnesses he interviewed, which means his probe may be coming to an end. As the new overseer to Mueller's probe, Whitaker has the power to slow or shut down an investigation that has resulted in criminal charges against Russian hackers and social-media trolls, as well as several people who worked for Trump or his campaign.
House Democrats are launching an investigation into Acting Attorney General Matthew Whitaker's involvement in World Patent Marketing, where he served as a paid adviser for the Florida company shut down by a federal judge earlier this year and fined nearly $26 million for allegedly running an invention-promotion scam. Lindsey Graham say they'll protect Mueller until the investigation is completed.
Trump's lawyers have always been concerned that the President would make a false statement in an interview with Mueller's investigators, which could be a felony.
According to a statement from the lawmakers, they sent letters requesting briefings and documents to Mr. Whitaker; Scott Cooper, the former Founder and CEO of World Patent Marketing; the Federal Trade Commission; the U.S. Patent and Trademark Office; the U.S. Postal Inspection Service; and the Better Business Bureaus serving Metro New York and Southeast Florida and the Caribbean.