Robert Mueller hopes to build a good enough obstruction case against Trump so that he may impeach the President for obstruction under the Logan Act.
The Logan Act states that only the president can represent the United States in a diplomatic capacity. Mueller hopes to prove that Trump’s transition team, and therefor the President as well, went too far in their conversations with Russia while Obama was still technically in office.
President Trump’s former national security adviser, Michael Flynn, acknowledged in court last week that he lied to F.B.I. investigators about his communications with the Russian ambassador to the United States in the run-up to Mr. Trump’s inauguration. While Mr. Flynn pleaded guilty to only one count of making materially false statements, his admissions leave little doubt that he also violated a the Logan Act. Mueller could try to leverage this so that Flynn tries to throw Trump under the bus to get himself out of hot water.
(There are more details to this conspiracy that you can read here.)
To be sure, the Republican-controlled Congress, not Mr. Mueller, decides who to impeach. Do you think Mueller will be able to drag Trump far enough through the mud to have a real shot at an impeachment case?