Beyond being led by a new FBI director, an independent investigation into possible Russian meddling could be handled by a special prosecutor, a special counsel or a new congressional committee or commission.
Congress could pass legislation requiring a special prosecutor
Congress has the ability to pass a law requiring that an independent special prosecutor be appointed under certain circumstances.
The catch? President Trump would need to sign that legislation, and extensive bipartisan support would be needed to override a veto.
What could that legislation look like? Lawmakers could reauthorize or pass a statute similar to one enacted in the wake of Watergate. The Ethics in Government Act of 1978 directed the attorney general to request a three judge panel to name a special prosecutor (later known as an independent counsel) if high level government officials faced allegations of certain federal offenses. The act expired in 1999.
Deputy Attorney General Rod Rosenstein could appoint a special counsel
Because Attorney General Jeff Sessions said he would recuse himself from any campaign-related Russia investigation, the decision to designate a special counsel falls to Rosenstein.
While a special counsel would have some day-to-day independence, he or she would have to answer to the administration via Rosenstein, who would have the power to overrule and fire the special counsel.
Rosenstein, who was overwhelmingly confirmed by the Senate two weeks ago, did not commit to appointing a special counsel in his confirmation hearing, despite Democrats' repeated efforts to make him do so.
An independent commission or select congressional committee
Congress can pass legislation to form an independent commission or select congressional committee to investigate. An independent commission would include respected figures outside government, while a select committee would include members of Congress. Any congressional committee could be given a broad mandate, unlimited resources and subpoena power.
Either kind of committee would put together a report summarizing its investigation but would not be able to bring criminal charges, as a special prosecutor or special counsel could.
Other tactics lawmakers could use
Democrats could pressure Rosenstein and Republicans with obstruction.
They could use procedural tactics to bring the Senate to a standstill: They could disrupt votes, committee meetings and block nominations and major legislation to pressure Republicans into supporting the naming of a special prosecutor. On Wednesday the ranking Democrats on several Senate committees used procedural measures to postpone hearings.
In the House, Democrats have more limited tools at their disposal, beyond forcing snap votes on various Trump-related provisions. They could staged protests, such as their gun-control sit-in tactic last year, and disrupt the House chamber. But it is unclear what impact that would have.