Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is completing a book about President Trump's national security team and policies. The opinions expressed in this commentary are his own. View more opinion articles at CNN.

(CNN) The whistleblower complaint, taken together with the transcript of President Trump's July 25 call with Ukrainian President Volodymyr Zelensky that was released earlier this week by the White House, explain why Nancy Pelosi is finally getting behind impeachment proceedings: A sitting president appears to have abused his office for political gain, and White House officials allegedly engaged in a cover-up of that effort.

Impeachment proceedings went ahead against President Richard Nixon because he engaged in a cover-up of a break-in at the Democratic Party office at the Watergate, of which he claimed to have no advance knowledge.

As is so often the case in Washington, it was the cover-up that rose to the level of a breach of public trust that was the grounds for impeachment proceedings against Nixon.

In Trump's case, Democrats in the House can credibly make the case that while in office Trump pressured a foreign leader to dig up dirt on a political opponent, Joe Biden, and his family. President Trump insists he did nothing wrong.

And then Trump officials, realizing that there was something unseemly about the call between Trump and the Ukrainian President made every effort, according to the whistleblower, to "lock down" the transcript of the call by removing the transcript from where these transcripts are generally stored and loading it on a separate computer system where only the most classified and sensitive communications are stored, despite the fact that there was no classified information in the call.

Read More