The American public needs to hear this as well. Amid the clamor of Mr. Trump’s perpetual outrages, Tuesday’s announcement signals: This moment is different. Pay attention. There are constitutional and national security issues at stake.

Mr. Trump has long argued — and continues to argue — that impeachment will benefit him politically. Many Democrats, including Ms. Pelosi, have not disputed that possibility. Pressing ahead with the proceedings despite such political uncertainty conveys Democratic leaders believe matters have reached the point where the costs of inaction are simply too high.

Having the controversy over Mr. Trump’s dealings with Ukraine serve as the trigger for the formal inquiry sends a specific warning to foreign governments: America does not welcome, nor will it tolerate, the meddling of other nations in its elections. This is an especially crucial point to make in light of Mr. Trump’s seeming refusal to confront Russia about its interference in 2016 and his public comment in June that he’d be open to foreign assistance in 2020. House leadership is making clear that it will take extreme action to prevent another assault on the democratic process.

This development also sends a message to whistle-blowers in this and future administrations. Central to the Ukraine controversy is the complaint from an administration whistle-blower, who followed the proper legal procedure for reporting concerns about sensitive information. The White House and Department of Justice tried to block those concerns from surfacing. Such an effort to undermine the confidential reporting process risks long-term damage to the national interest. As noted by Michael McFaul, an ambassador to Russia during the Obama administration, this could convince other potential whistle-blowers that the system doesn’t work, prompting them to resort to illegal leaking, à la Edward Snowden.

House Democrats are also making clear to the courts that they are conducting an official impeachment inquiry, which could have concrete legal ramifications. Some experts contend that an impeachment inquiry gives Democrats more investigative power than regular oversight proceedings. That is why, in their legal battle with the administration over documents and testimony, Democrats had been referring to their investigation as an impeachment inquiry, even as House leadership declined to use the term. The administration was using the latter’s reticence as a legal argument against cooperating with investigators. Ms. Pelosi has now ended any debate over the nature of the inquiry.