WASHINGTON — The House of Representatives voted on Wednesday to transmit two articles of impeachment against President Trump to the Senate, sending the president and his party into uncharted territory in a deeply divisive trial fraught with history and political risk.

In a choreographed ritual, the House formally appointed seven Democrats to serve as impeachment managers prosecuting the case before the Republican-controlled Senate. The group silently marched two charges of high crimes and misdemeanors, encased in slim blue folders, across the Capitol to set in motion the third presidential impeachment trial in American history.

The proceedings commencing on Thursday will play out in a Capitol already rived by politics during a contentious election year. Among the senators who will be sitting in judgment of Mr. Trump will be four Democrats who are running for president, juggling their campaigns to defeat him with their duties as jurors.

The trial is laden with peril for Mr. Trump. He will face weeks of public discussion of the allegations that he solicited foreign help in the 2020 presidential election, abusing the power of his office and obstructing a congressional inquiry in the process. But the president is almost certain to cast his likely acquittal as a complete exoneration and turn the considerable apparatus of his campaign to stoke public outrage.

Democrats believe the proceeding will put pressure on Republicans — particularly those facing tough re-election challenges — to condemn Mr. Trump or risk being cast as an apologist for the president. Yet they, too, risk a backlash if voters sour on the impeachment effort, dismissing it as a political stunt.

“We are here today to cross a very important threshold in American history,” Speaker Nancy Pelosi said as she spoke on the House floor before a vote to transmit the articles. Regardless of the outcome, she added, Mr. Trump would be “impeached for life.”

The House impeached Mr. Trump last month on charges of abuse of power and obstruction of Congress, formally accusing him of pressuring Ukraine to investigate his political opponents while withholding as leverage a nearly $400 million package of military assistance and a White House meeting with the country’s president.

Credit…Erin Schaff/The New York Times

Wednesday’s vote, almost exactly a month later, was largely along party lines. Only one Democrat, Representative Collin C. Peterson of Minnesota, joined every Republican in voting “no.”

The action deputized the seven impeachment managers who will present the House’s case. The group, mostly lawyers, included a former police chief and reflected the geographic and demographic diversity of the Democratic caucus, with three women, two African-Americans and one Latina.

Hours later, after an elaborate signing ceremony, they walked two by two through a hushed Capitol, carrying the articles — embossed with a gold seal and bearing the signatures of Ms. Pelosi and the House clerk — to the Senate. More than two dozen Democrats, but only two Republicans — Senator Mitch McConnell of Kentucky, the majority leader, and Senator Kevin Cramer of North Dakota — sat in the chamber awaiting their arrival.

At the White House, the president denounced the effort to remove him anew as a “hoax” that Republicans would soon debunk — “let’s take care of it,” he told members of the House attending a ceremony in the East Room not long before the vote. Senior administration officials projected confidence, calling theirs an “easy case” and predicting that the Senate would acquit the president in two weeks without hearing from any witnesses.

But the path ahead in the Senate was far from clear, as the normally staid chamber braced for a potentially fractious proceeding and some Republicans agitated for a lengthier and more thorough process.

Senator Susan Collins, Republican of Maine, confirmed that she and a small group of fellow Republicans — Senators Lisa Murkowski of Alaska, Lamar Alexander of Tennessee and Mitt Romney of Utah — had worked with Mr. McConnell to ensure there would be a vote on whether to call witnesses or collect new documentary evidence, once each side had presented its case and senators had a chance to ask questions.

“It’s important that we have an up-or-down vote on the issue of subpoenaing witnesses and documents,” Ms. Collins told reporters, “and I’ve worked very hard to get that included, with all my colleagues that I’ve mentioned, into the governing resolution.”

The president himself is “of two minds” on the matter, said Senator Rand Paul, Republican of Kentucky, saying that Mr. Trump was vacillating between wanting a prolonged trial with a robust public defense of his actions and a quick dismissal.

“I think he feels that he’s been unjustly accused, and would like to present his side of it,” Mr. Paul said in an interview on Wednesday. But he added, “I think it’s hard to sort of turn down the idea that we could have a motion to end this thing.”

Mr. McConnell accepted the articles late Wednesday afternoon and invited the managers to exhibit them at noon Thursday. The Senate will then summon Chief Justice John G. Roberts Jr., who will preside over the trial, to administer an oath binding senators to render “impartial justice” and issue a summons asking the president to answer to the charges against him.

The trial, however, will not begin in earnest until Tuesday, with debate on a resolution to set the rules followed by opening arguments by the House managers and the president’s legal team.

After weeks of sharp words, Mr. McConnell sounded an unusually somber note on the Senate floor, in a nod to the weight of the occasion.

“This is a difficult time for our country, but this is precisely the kind of time for which the framers created the Senate,” he said. “I’m confident this body can rise above the short term-ism and factional fever and serve the long-term best interests of our nation. We can do this and we must.”

Ms. Pelosi withheld the articles from trial for nearly a month to try to increase pressure on Republicans like Ms. Collins to vote for new evidence. Democrats argued aggressively that a trial without it is tantamount to an extension of Mr. Trump’s attempts to cover up his actions.

Pressing their case, just as lawmakers were preparing to transfer the articles across the Capitol, the House made public another tranche of newly received texts, calendar entries, notes and other records illustrating aspects of Mr. Trump’s pressure campaign on Ukraine that could be used by prosecutors in the trial.

They came from Lev Parnas, an associate of Mr. Trump’s personal lawyer Rudolph W. Giuliani, and appeared to detail attempts to dig up dirt in Ukraine on Mr. Trump’s political rivals, actions that lie at the heart of the House’s case. In an interview with The New York Times, Mr. Parnas asserted that Mr. Trump had been fully aware of his efforts, though the two men never spoke about it.

“Time has been our friend in all of this because it has yielded incriminating evidence, more truth into the public domain,” Ms. Pelosi told reporters.

To lead the team of managers to prosecute the case, Ms. Pelosi turned to one of her most tested deputies, Representative Adam B. Schiff of California, a former federal prosecutor who oversaw the House’s Ukraine inquiry as the chairman of the Intelligence Committee.

The group also includes Representatives Jerrold Nadler of New York, the chairman of the Judiciary Committee; Zoe Lofgren of California, a veteran of three presidential impeachment debates; Hakeem Jeffries of New York, a fast-rising Democratic leader and the party’s messaging chief; Val B. Demings of Florida, a former police chief; and Jason Crow of Colorado and Sylvia R. Garcia of Texas.

Several of the lawmakers have courtroom experience of some kind, a quality Ms. Pelosi said she had sought. Two, Mr. Crow and Ms. Garcia, are both first-term members, and Mr. Crow has military experience allowing him to speak to the national security implications of some of Mr. Trump’s actions.

The managers met for the first time as a group on Wednesday to discuss strategy in the basement chambers of the Intelligence Committee, where the impeachment inquiry unfolded last fall.

In the coming days and weeks, the managers will try to lift their arguments against Mr. Trump above ordinary politics. Like Mr. Trump’s defense team, they will aim their case not only at the 100 senators gathered at trial but at voters who will go to the ballot box in November and serve as the ultimate appeals court on Mr. Trump’s presidency.

They will seek to recreate the highlights of the two-month investigation of the Ukraine matter, relying on testimony from more than a dozen senior American diplomats and White House officials who defied the president’s orders not to cooperate.

Those witnesses outlined a broad campaign by Mr. Trump to use the levers of his government to exert pressure on Ukraine to publicly announce investigations into former Vice President Joseph R. Biden Jr. and claims that the Democrats colluded with Ukraine in the 2016 election. The president, the House concluded, ultimately withheld the military aid earmarked for Ukraine and a coveted White House meeting for its new leader as leverage.

But the managers must also contend anew with Mr. Trump’s successful stonewalling. His across-the-board refusal to produce documents or top aides to investigators resulted in the second impeachment charge, obstruction of Congress, but also holes in the factual record that the senators have already made clear they will call into question.

“This trial is necessary because President Trump gravely abused the power of his office when he strong-armed a foreign government to announce investigations into his domestic political rival,” Mr. Nadler said during a brief debate on the House floor before the vote. “But we still have not heard the whole truth.”

Republican leaders on both sides of the Capitol appeared unmoved. Mr. McConnell called the House’s three-month Ukraine investigation “a pale imitation of a real inquiry.” Representative Kevin McCarthy, Republican of California and the minority leader, called the whole process a “national nightmare.”

Emily Cochrane and Annie Karni contributed reporting from Washington, and Maggie Haberman from New York.