Schumer sent Trump a letter on Friday warning that Whitaker's appointment ignored the line of succession within the Justice Department and "potentially violated the Appointments Clause of the Constitution" by putting someone who had not been previously confirmed by the Senate into the Justice Department spot. 
"Mr. Whitaker is a political appointee who is not serving in a Senate confirmed position in the Justice Department.  I am not aware of any precedent for appointment of an official who has not been confirmed by the Senate to serve as acting Attorney General," Schumer wrote in the letter. 

He added that the decision to name Whitaker, who was previously Jeff SessionsJefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE's chief of staff, "raises a number of questions regarding the propriety and even the constitutionality of that action" and that he wants answers about Trump's "motivations" behind the decision.

Trump announced on Wednesday that he was ousting Sessions, who had been a frequent public punching bag, and replacing him, temporarily, with Whitaker. The White House hasn't given any specifics on when they will nominate a permanent attorney general and it's unlikely that the Senate would have enough time to confirm a nominee this year. 

Schumer said he couldn't see a "legitimate reason" for appointing Whitaker when Rosenstein had been Senate confirmed and that the decision was "clouded by unresolved constitutional questions."

Schumer is asking Trump to explain why he leapfrogged over Rosenstein, if he discussed the decision of who to appoint as acting attorney general with anyone else in the administration or if he asked for advice from the Justice Department's Legal Counsel. 
He also wants to know if he discussed Mueller's investigation with Whitaker before he was appointed as acting attorney general or if he made Whitaker take a "pledge of loyalty."