Thursday was a day of pomp and ceremony in the Senate at the opening of the impeachment trial of President Trump. But apart from all the scripted theater, the formalities marked a new and important test for the Republicans who control the upper chamber and therefore what kind of trial the country sees and what history remembers of those who sat in judgment.

The challenge for Republicans is not the question of the ultimate disposition of the case against Trump contained in the two articles of impeachment read into the record on the Senate floor at midday Thursday. That appears to be a foregone conclusion, given the partisan makeup of the Senate.

Instead it is the question of how they will address and digest evidence of the president’s actions with regard to Ukraine that has come out over the past four months. It is not just the president’s legacy that will be affected by the conduct of the proceedings over the next few weeks. It will also be the legacy of the Republican senators and their party.

Will they follow the lead of their House colleagues, who in the face of damning testimony embraced the president’s explanations, that his interactions with Ukraine were “perfect,” and that he was acting in the interests of the country rather than for personal political gain? Or will they judge him more independently, and critically, even if eventually stopping short of casting guilty votes?

The House managers, who will begin to present their case Tuesday, will come with the evidence developed during weeks of testimony and debate that resulted in the party-line vote to impeach Trump at the end of December. That evidence includes testimony from a series of career Foreign Service and national security officials outlining the months-long effort to pressure Ukraine.

But that won’t be the only thing that senators sitting as jurors will have to take under consideration, as it is likely that additional evidence will emerge from outside the chamber that will weigh on their decision-making. Already, in the month since the House voted to impeach Trump, there have been a series of revelations and statements that pose new challenges for the president’s defense team and, importantly, for those in his party.

Hours before Chief Justice John G. Roberts Jr. was sworn in as the presiding officer for the trial, and before he in turn swore in the members of the Senate, the Government Accountability Office issued a report stating that the White House had broken the law by withholding $391 million in military aid for Ukraine that had been authorized by Congress. A spokeswoman for the Office of Management and Budget took issue with that finding.

The GAO report followed the release of documents given by Lev Parnas, an associate of Rudolph W. Giuliani, the president’s personal attorney, to House investigators and television interviews Parnas gave Wednesday. Those interviews and the documents further highlighted Giuliani’s role in Ukraine, including the efforts to force the removal of Marie Yovanovitch as U.S. ambassador to Ukraine. Parnas’s description of events also tied the president more directly to the activity. He is under indictment in federal court and his word must be viewed with skepticism, but what he has said and the documents he has offered further raise the stakes.

Shortly after the House’s impeachment vote, new emails from within the administration surfaced, including one from Michael Duffey, a senior official in the Office of Management and Budget, directing Pentagon officials to put a hold on the aid to Ukraine. The email was sent less than two hours after a Trump phone call with Ukrainian President Volodymyr Zelensky in which Trump had requested “a favor”: an investigation into a discredited theory about Ukrainian interference in the 2016 election as well as into former vice president Joe Biden and his son Hunter.

There was also one other surprise during the interregnum between the House vote and this week’s formalities. That was the statement from John Bolton, the former national security adviser, who said that, if subpoenaed, he was prepared to testify at the Senate trial. As one of a handful of White House officials with direct knowledge of the events under scrutiny, Bolton was offering something that has made Republican senators squirm.

It will take 51 votes for any witnesses to be allowed into the Senate trial, which means support from at least four Republicans. When it happens, as seems likely, the vote will trigger another partisan battle over who should be called, particularly what witnesses the president’s defenders might demand — such as Biden or his son. That fight will not come for some days, not until after the House managers and the president’s defense team present their cases.

Members of the Senate took an oath to render impartial justice, but in the hyperpartisan climate that exists today, many lawmakers already have declared their verdicts. Some Democrats, including some of the senators seeking the presidential nomination, have said they have seen enough to persuade them that Trump should be convicted and removed from office. On the other side are Republicans who have declared Trump not guilty, and Senate Majority Leader Mitch McConnell (R-Ky.) said weeks ago he was working closely with the White House in preparation for the trial.

But if, as everyone expects, the verdict is a predictable acquittal on what is likely to be a vote that runs closely if not exclusively along party lines, the question of how seriously the evidence is weighed and examined is another matter. The president would have preferred a breezy dismissal, though that was never realistic. On Thursday, shortly after the senators had been sworn, he said he hopes for a quick trial, declaring the whole matter a “phony hoax.”

Many of Trump’s loyalists believe Democrats have been looking for any reason to bring down the president and treated the current impeachment process accordingly. What they did not do in the House was to effectively rebut the testimony of those career officials. Instead they challenged that evidence on the basis that it was not firsthand knowledge, even as the White House blocked those with such knowledge from testifying.

The Senate trial will give the president an opportunity for his team of lawyers to present a defense. Whether they accede to witnesses, particularly the White House officials who have not yet been heard from, is another matter.

Being a Republican during the Trump presidency demands much. He is quick to anger at any Republican who strays from absolute loyalty and at times has sought to punish those who have. Few have had the wherewithal to question him, and they have generally paid a price. Their examples have shaped the behavior of others in the party.

That’s a major difference between this impeachment proceeding and the one two decades ago involving President Bill Clinton. That Senate trial ended in an acquittal, largely on party lines. But the year-long episode also included condemnations of the president’s behavior by members of his own party, both what he did and that he lied about it.

If Clinton’s Democratic colleagues did not believe his conduct ultimately warranted removal from office, they nonetheless found it worthy of rebuke. That is one way to measure how those in the president’s party handle the allegations contained in the two articles of impeachment against Trump and ultimately the meaning of “impartial justice” as defined by each individual senator.