Its not a legislative thing. Several committees within the legislative branch are investigating Russian collusion and other activities related to Trump. The committees have broad legal and judicial discretion to subpoena witness and have said witnesses tell the truth, and at the end of this, vote to impeach the President. Ditto with the independent commission. Furthermore, once it is in the public that Trump attempted to obstruct justice, politicians will be under immense pressure to turn up the heat on Trump, which could result in impeachment.
To be clear, what Trump has done is easily an impeachable offense.