Editor’s Note: 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.”

In other recent hearings, Democrats have gone after the President’s missing tax returns, opening up the possibility of getting a better look at his earnings and whether his own self-interest might have influenced any policy decisions. California Democrat Adam Schiff is also going to make sure that the House takes a bite into the investigations into Russian interference in the 2016 election. Rather than leaving the matter to Mueller, the House will finally be joining the genuine investigative process.

Democrats are also forcing discussions about health care policy after the midterm elections showed it was critical to voters. Democrats have convened a series of hearings into the Affordable Care Act as part of a legislative push to protect Americans with pre-existing conditions. Virginia Democrat Robert Scott, who was perhaps happy to be talking about something other than the chaos in Richmond, led the Education and Labor Committee into a discussion about the need to ensure the protections afforded under the ACA are not reversed.

The young maverick Alexandria Ocasio-Cortez is also shaking Washington out of its policy slumber by generating excitement for a “Green New Deal.” While many Democratic leaders think the legislation she and Sen. Edward Markey have proposed is far too ambitious and unrealistic, she has managed to get a big idea on the table and accelerate the debate over this crucial issue. As the concern for climate change grows, House Speaker Nancy Pelosi has also set up a select committee to examine the ways to address this challenge while the Committee on Natural Resources held its own hearings this week.

As if all this were not enough, the House Judiciary Committee opened its hearings into gun violence prevention. Doing something about guns is extremely popular among educated suburban and female voters – a key constituency in 2020. The frustration building among Republicans was evident when Rep. Matt Gaetz tried to have two fathers of Parkland, Florida, shooting victims removed from the hearing. The moment perfectly captured the dynamics. Republicans are desperate to have any discussions about the crisis of firearms silenced. Unfortunately for them, they no longer hold the majority in the House.

The effort to stay in control of the national conversation will not be easy. Trump and the Republicans in Congress will do everything they can to throw the Democrats off track. During the partial government shutdown last month, Trump showed how the perpetual chaos that dogs his presidency has the ability to shut down all other debates.

Democrats will also have plenty of opportunities to shoot themselves in the foot. The recent scandals engulfing the governor, lieutenant governor and attorney general in Virginia have been devastating in a state that looked to be leaning blue.

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    But the challenges do not eclipse the successes Democrats have enjoyed in the past week. The conversation in Washington has shifted to issues that are favorable to Democrats. They have also done so much to drown out Trump’s Twitter feed. If Democrats can continue applying pressure on the President and his allies, and raise important policy discussions, they might just do more than any single presidential candidate in bolstering the party’s chances for reclaiming the White House in 2020.