Julian Zelizer is a professor of history and public affairs at Princeton University and co-author with Kevin Kruse of the new book "Fault Lines: A History of the United States Since 1974." The opinions expressed in this commentary are his own. View more opinion at CNN.

(CNN) Democrats finally have a bully pulpit of their own: the House of Representatives.

Julian Zelizer

Control of the House is an awesome source of power. Within one month of officially retaking the lower chamber, the Democrats have managed to do something that proved to be almost impossible in the past two years -- they have constrained President Donald Trump's ability to dominate the national conversation. For the first time, his endless stream of tweets has been pushed a little further into the background.

Democrats have used their newfound power in the House to focus on investigations surrounding Trump, while forcing policy discussions on health care, climate change and gun control. As a result, the media and the public are talking about these issues instead of constantly responding to every provocation that comes from the President.

On Friday, Democrats were able to raise their concerns and ask whether the President and the Justice Department have been trying to stifle special counsel Robert Mueller's investigation. Acting Attorney General Matthew Whitaker testified before the House Judiciary Committee and literally tried to silence Chairman Jerrold Nadler by telling him his five minutes were up . By plowing forward with his question, Nadler showed audiences that Whitaker and the administration are now in the hot seat.

Trump responded by calling the Democrats "vicious" on Twitter and claimed they "totally showed their cards for everyone to see."

Read More