WASHINGTON — For only the fourth time in its history, the House voted Thursday to initiate impeachment against a president of the United States.

As a technical matter, the resolution was a dry set of rules for the public phase of an investigation into President Donald Trump that has been under way informally almost since Democrats took control of the House in January.

But on a political level, the floor fight over it was nasty, brutish and relatively short — just over an hour — ending in a nearly perfect party-line vote. Two Democrats voted "no," and one independent, Rep. Justin Amash of Michigan, an independent who left the GOP essentially in protest over his party's allegiance to Trump, voted in favor.

The contours of Thursday's debate, and the vote totals on each side, set a baseline from which the two parties will battle over the coming weeks as the Democrats hurtle almost inevitably toward bringing articles of impeachment against Trump. They now know they still have work to do to force Republicans to cross the partisan aisle, by applying public pressure — a prerequisite to eventually sway senators — and Republicans now know that most politically vulnerable Democrats are unafraid of the consequences of pursuing impeachment.

House Speaker Nancy Pelosi, D-Calif., was able to lock in votes from swing-district lawmakers after the probe transformed late last month from a sprawling look into Trump's 2016 campaign, his tax records and the financial benefits that have accrued to his family's business during his presidency to one focused primarily on whether he corruptly abused his power over foreign aid to Ukraine to help himself politically.

Pelosi said Thursday that lawmakers are called upon to "protect and defend the Constitution" so that "no president, no matter whoever he or she may be in the future, could decide that Article II says they can do whatever they want."

Republicans chose to play defense by going on offense. Rep. Liz Cheney of Wyoming, chair of the Republican Conference, and Rep. Devin Nunes of California, the top-ranking Republican on the Intelligence Committee, said it wasn't Trump who was imperiling the Constitution.

"They have absolutely no right to talk about threats to this nation if they are diverting the full attention, resources and focus of the House Intelligence Committee onto a sham political process," Cheney, a daughter of former Vice President Dick Cheney, said of Democrats. "They will be held accountable by history."

GOP lawmakers continued to assail the details of the process, faulting Democrats for setting up public Intelligence Committee hearings in which Chairman Adam Schiff of California, has final say over which witnesses can be subpoenaed by the minority and whether White House lawyers can attend sessions.

House Minority Leader Kevin McCarthy, R-Calif., launched his most aggressive defense of Trump yet, moving from simply blasting the impeachment process into the territory of justifying the president's conduct — an area that has been a no-man's land for many of his colleagues.

He said the Democratic-led House is "abusing its power to discredit democracy" and framing "the president's legitimate actions as an impeachable offense."

Those actions amount to an initiative, carried out by the president, top political lieutenants within and outside government, and career federal officials who were either witting or unwitting players, to pressure Ukraine's new president, Volodymyr Zelenskiy, into opening a pair of investigations by withholding American financial aid and the imprimatur of a meeting with Trump.

According to a White House summary of his July phone call with Zelenskiy and witness testimony provided to the House Intelligence Committee, Trump sought publicly announced probes into Vice President Joe Biden and his son Hunter, as well as the whereabouts of a conspiracy-theory-concocted server that supposedly held evidence that Democrats had framed Russia for interference in the 2016 presidential election.

Trump also publicly called on Ukraine and China to investigate the Bidens.

Few Republicans other than McCarthy have gone so far as to describe Trump's pursuit of foreign investigations into Biden, who is the polling leader for the Democratic presidential nomination, or his pause on foreign aid appropriated by Congress for political or policy reasons as "legitimate."

Many have said those actions fall short of their definition of impeachable offenses, but they have been wary of approving of the behavior. And some — most notably 2016 Republican presidential nominee Mitt Romney, now a senator from Utah — have slammed Trump for his conduct.

"By all appearances, the president's brazen and unprecedented appeal to China and to Ukraine to investigate Joe Biden is wrong and appalling," Romney said recently.

Thursday's vote showed both that Democrats are now almost entirely unified in the view that it's time for a public airing of Trump's actions — affirming that they believe they won't be punished heavily at the polls for rebuking his conduct — but also that Republicans feel safe in defending him.

Whether that changes between now and whenever Democrats bring articles of impeachment to the House floor will depend on their ability to make the case that the president has not only befouled his office but must be removed from it. The bar for that — as Thursday's vote, the Republican majority in the Senate and the Constitution's two-thirds-of-the-Senate vote requirement for removal suggest — is very high.