Keith Whittington's op-ed in The New York Times, Trump's Scary Precedent: Scorning Congress, makes the case that the obstruction of justice charge in the articles of impeachment (Article 2) reflects critical interference with Congress's duties:
There may be circumstances in which Congress is willing to tolerate such obstruction. But when there are reasonable fears that the obstruction is itself intended to hide abuses of power, then Congress cannot and should not countenance such interference with its duties.
Keith Whittington is William Nelson Cromwell Professor of Politics and a CSDP faculty associate at Princeton University. He is the author of numerous books, including Speak Freely: Why Universities Must Defend Free Speech, as well as Constitutional Construction: Divided Powers and Constitutional Meaning, and Constitutional Interpretation: Textual Meaning, Original Intent, and Judicial Review, and Political Foundations of Judicial Supremacy: The Presidency, the Supreme Court, and Constitutional Leadership in U.S. History.