President Trump in the Oval Office of the White Houseon May 3. (Jabin Botsford/The Washington Post)

Julian E. Zelizer is a professor of history and public affairs at Princeton University.

As President Trump moves in aggressive fashion to block and obstruct congressional investigations, House Democrats seem unsure what to do. The president, who repeatedly refused to cooperate with Robert S. Mueller III’s investigation, is blocking testimony, resisting subpoenas, claiming executive privilege and continuing to use his Twitter bully pulpit to dirty public opinion about the legitimacy of anyone asking serious questions about what he is up to. “Trump is goading us to impeach him,” House Speaker Nancy Pelosi (D-Calif.) said.

Yet Democrats are scared about what comes next. Even as party leaders speak of a “constitutional crisis” and a president who is “becoming self-impeachable,” House Democrats have not yet started impeachment proceedings, and many would rather continue with alternative forms of oversight that fall short of the full power Congress has over the White House.

Given the abuses of power by Trump, why are Democrats so worried about starting a process to see whether articles of impeachment are warranted?

The party has good reason to be leery. The Democrats fear triggering a destabilizing political process that should be used only as a mechanism of last resort. Because Republicans in Congress have decided to put partisanship above country on all matters related to Trump, Democrats would go into this process without any chance of bipartisan support. Pelosi and other Democrats want to avoid doing what they believe Republicans did in 1998: They allowed impeachment to become a tool of partisan warfare. Better to wait and be patient, they say, until there is no choice or, better yet, until he loses his position.

Indeed, political concerns are also at work. Sensing that they can win back the White House in 2020, many Democrats don’t want to take a single step that might jeopardize their standing in 2020 by fueling a pro-Trump backlash to Democratic investigations. The worst of all worlds, they say, is to mobilize the president’s base, lose valuable time when they can be discussing key policy issues, and then watch Senate Republicans exonerate him regardless of what the investigations find.

All of these concerns are reasonable.

But sometimes parties can be too scared about using their power when it is necessary to do so, and we keep getting closer to such a moment every day. In every era, there are historic turning points when a party in power finds itself saddled with responsibility — and the party will be judged in the long term on whether it lives up to that responsibility. The question for Democrats is whether it is now time to respond to the dangers that the democracy faces and to lead, rather than simply follow, public opinion.

While it is impossible to game out everything that might unfold, it is easy to see what allowing the president’s continuing abuse of power is doing to our long-cherished norms and precedents. We have in fact seen this every day since January 2017. What keeps becoming clearer is how each act of abusive power begets another. Trump and his team keep getting bolder in what they are willing to do to hold and consolidate power, such as Rudolph W. Giuliani, his lawyer, pressing Ukraine to dig up dirt, real or imaginary, on former vice president Joe Biden, the president’s leading Democratic opponent.

At the same time that Democrats understand defeating Trump in 2020 is a better resolution to the constitutional crisis than proceeding with impeachment, they also need to think about the possibility that he could win reelection. With a relatively strong economy and solid approval ratings within the GOP, there is nothing inevitable about Democrats making Trump a one-term president.

House Democrats need to remain extremely proactive in their current investigations. They must insist on subpoenaing Attorney General William P. Barr to submit to questions by congressional lawyers, and they must respond to each invocation of executive privilege so that the courts become immediately involved. They should adopt the proposal by Rep. Adam B. Schiff (D-Calif.) to levy fines on officials who attempt to evade subpoenas. But most important is the need to consider starting impeachment proceedings, with all the powers and privileges that come with this process, so that both parties can formally go on record as to where they stand on the president’s abuse of power.

If House Democrats decide to let the abuse of power stand, this form of presidential authority would be inscribed into the system as the new normal. Just as important, in the short term it would become the foundation upon which a second-term President Trump could use his executive power in ways that we can’t even imagine today as he attempts to demolish the social safety net, unleash market forces on the climate, take apart vital international alliances and undercut the health of our democratic processes.

If House Democrats stand by as institutional norms crumble, at some level their party will go down in the history books alongside the Republicans as complicit in cementing the Trumpian era of presidential power.