Former President Donald Trump on Friday sued the House committee investigating the Jan. 6 riot, arguing its subpoena seeking his testimony and documents tied to the Capitol attack was invalid.A lawyer for Trump, David A. Warrington of the Dhillon Law Group, said in a statement: “Long held precedent and practice maintain that separation of powers prohibits Congress from compelling a President to testify before it.""President Trump joins Presidents of both parties in insisting that the legislative branch honor the boundaries set forth in the Constitution, instead of catering to base partisan impulses,” Warrington added.In remarks at the University of Chicago Institute of Politics earlier Friday, committee vice chair Rep. Liz Cheney, R-Wyo., said Trump “has a legal obligation and if he fails to meet it the committee will consider next steps.”The committee subpoenaed Trump last month for any communications he may have had regarding extremist groups involved in the riot and any attempts of his in the past year to contact witnesses testifying before the committee.Trump's lawyers on Friday appeared to give a nod toward those arguments when they wrote: "The broad scope of the Subpoena’s request for documents and testimony threatens to force President Trump to reveal the inner workings of his Presidential campaign, including his political beliefs, strategy, and fundraising.""