WASHINGTON >> House Democrats moved aggressively to draw up formal articles of impeachment against President Donald Trump on Thursday, with Speaker Nancy Pelosi saying he “leaves us no choice” but to act swiftly because he’s likely to corrupt the system again unless removed before next year’s election.

A strictly partisan effort at this point, derided immediately by Trump and other leading Republicans as a sham and a hoax, it is a politically risky undertaking. Democrats say it is their duty, in the aftermath of the Ukraine probe, while Republicans say it will drive Pelosi’s majority from office.

Congress must act, Pelosi said. “The democracy is what is at stake.”

“The president’s actions have seriously violated the Constitution,” she said in a somber address at the Capitol. “He is trying to corrupt, once again, the election for his own benefit. The president has engaged in abuse of power, undermining our national security and jeopardizing the integrity of our elections.”

Trump has insisted he did nothing wrong. He tweeted that the Democrats “have gone crazy.”

At the core of the impeachment probe is a July phone call with the president of Ukraine, in which Trump pressed the leader to investigate Democrats, including political rival Joe Biden. At the same time the White House was withholding military aid from Ukraine, an ally bordering an aggressive Russia.

Drafting articles of impeachment is a milestone moment, only the fourth time in U.S. history Congress has tried to remove a president, and it intensifies the rigid and polarizing partisanship of the Trump era that is consuming Washington and dividing the nation.

The speaker delivered her historic announcement in solemn tones at the Capitol, drawing on the Constitution and the Founding Fathers in forcefully claiming Congress’ oversight of the president in the nation’s system of checks and balances. Democrats are already beginning to prepare the formal charges, pushing toward House votes, possibly before Christmas.

“Sadly, but with confidence and humility, with allegiance to our founders and a heart full of love for America, today I am asking our chairmen to proceed with articles of impeachment,” Pelosi said.

Seemingly eager to fight, Trump tweeted that if Democrats “are going to impeach me, do it now, fast.” Though he has fought the House investigation, trying to bar current and former officials from testifying, he said he now wants to move on to a “fair trial” in the Senate.

Approval of articles of impeachment is considered likely in the Democratic-majority House. Conviction in a following trial in the Republican-dominated Senate seems very unlikely.

Once reluctant to pursue impeachment, warning it was too divisive for the country and needed to be bipartisan, Pelosi is now leading Congress into politically uncertain terrain for all sides just ahead of the election year.

Republican are standing lockstep with Trump, unswayed by arguments that his actions amount to wrongdoing, let alone impeachable offenses. That is leaving Democrats to go it alone in a campaign to consider removing the 45th president from office.

At a town hall late Thursday on CNN, Pelosi said she would have no regrets if impeachment ended up helping Trump’s reelection effort. “This isn’t about politics at all,” she said. “It’s about honoring our oath of office” — to defend the Constitution.

Pelosi has emphasized the Russia connection, from special counsel Robert Mueller’s probe into 2016 election interference to the president’s phone call this summer with Ukrainian President Volodymyr Zelenskiy that set off alarms in Washington.

Russia and President Vladimir Putin benefited most from Trump’s actions toward Ukraine, she said.

“All roads lead to Putin. Understand that,” she declared at a morning news conference at the Capitol. “That was the a-ha moment.”

She spoke solemnly and calmly, but that changed when she was asked as she was leaving if she hates Trump.

Pelosi stiffened, returned to the podium and responded sharply that the president’s views and politics are for the voters to judge at elections but impeachment “is about the Constitution.” She said that as a Catholic, she does not hate the president but rather is praying for him daily.

Trump quickly tweeted back that he didn’t believe her.

Trump’s allies argue that voters, not lawmakers, should decide the president’s future. But Democrats say the nation cannot wait for the 2020 election, alleging Trump’s past efforts to have foreign countries intervene in the presidential campaign are forcing them to act to prevent him from doing it again. Pelosi said the still-anonymous whistleblower’s complaint about Trump’s Ukraine call changed the dynamic, creating the urgency to act.

The number of articles and the allegations they will include will be both a legal and political exercise for the House committee chairmen, who will be meeting privately. They must balance electoral dynamics while striving to hit the Constitution’s bar of “treason, bribery or other high crimes and misdemeanors.”

Pulling from the House’s 300-page investigation of the Ukraine matter, Democrats are focusing on at least three areas — abuse of power, bribery and obstruction — that could result in two to five articles, they say.

They argue that Trump abused the power of his office by putting personal political gain over national security interests; engaging in bribery by holding out $400 million in military aid that Congress had approved for Ukraine; and then obstructing Congress by stonewalling the investigation.

Some liberal Democrats want to reach further into Trump’s actions, particularly regarding the findings from special counsel Mueller’s report on Russian interference in the 2016 election. That could produce an additional article of obstruction not only of Congress, but also of justice.

But more centrist and moderate Democrats, those lawmakers who are most at risk of political fallout from the impeachment proceedings, prefer to stick with the Ukraine matter as a simpler narrative that Americans can more easily understand.

The GOP leader of the House, Kevin McCarthy, said Pelosi is more concerned about tearing the president down than building the country up. Senate Majority Leader Mitch McConnell, R-Ky., criticized Democrats for focusing on impeachment over other issues, though many House-passed bills are waiting for action in his chamber. “It’s all impeachment, all the time,” he said.

At the White House, press secretary Stephanie Grisham tweeted that Pelosi and the Democrats “should be ashamed.”

House members are preparing to vote on the articles of impeachment in the Judiciary Committee, possibly as soon as next week. The committee set a Monday hearing to receive the Intelligence Committee’s report outlining the findings against the president.

The House is expecting a full vote by Christmas. That would send the issue to the Senate for a trial in the new year.