WASHINGTON — The U.S. Senate plunged into President Donald Trump's impeachment trial with Republicans abruptly abandoning plans to cram opening arguments into two days but solidly rejecting for now Democratic demands for more witnesses to expose what they deem Trump's "trifecta" of offenses.

Trump himself said Wednesday he wants top aides to testify, but qualified that by suggesting there were "national security" concerns to allowing their testimony.

"We have a great case," Trump said at a global economic forum in Davos, Switzerland. In a press conference before returning to Washington, Trump said his legal team was doing a "very good job."

He appeared to break with Republicans efforts to block Democratic motions to immediately call witnesses and subpoena documents. Instead, Trump said he'd like to see aides, including former national security adviser John Bolton, Secretary of State Mike Pompeo, and acting chief of staff Mick Mulvaney, testify as witnesses

Trump said he'd leave the "national security" concerns about allowing their testimony to the Senate.

Tuesday's daylong session started with the setback for Republican Senate leader Mitch McConnell and the president's legal team, but it ended near 2 a.m. Wednesday with Republicans easily approving the rest of the trial rules largely on their terms.

Senate Majority Leader Mitch McConnell walked back to the Senate chamber during the impeachment trial of President Donald Trump on Tuesday.

With the rules settled, the trial is now on a fast-track. At issue is whether Trump should be removed from office for abuse of power stemming from his pressure on Ukraine to investigate Democratic rival Joe Biden and Biden's son Hunter as Trump was withhold aid to the country, and for obstructing Congress' ensuing probe.

Chief Justice John Roberts gaveled open the session, with House prosecutors on one side, Trump's team on the other, in the well of the Senate, as senators sat silently at their desks, under oath to do "impartial justice." No cellphones or other electronics were allowed.

As the day stretched deep into the night, lawyerly arguments gave way to more pointedly political ones. Tempers flared and senators paced the chamber. Democrats pursued what may be their only chance to force senators to vote on hearing new testimony.

After one particularly bitter post-midnight exchange, Roberts intervened, taking the rare step of admonishing both the Democratic House managers prosecuting the case and the White House counsel to "remember where they are."

"I think it is appropriate at this point for me to admonish both the House managers and the president's counsel in equal terms to remember that they are addressing the world's greatest deliberative body," the usually reserved Roberts said. He told them that description of the Senate stemmed from a 1905 trial when a senator objected to the word "pettifogging," because members should "avoid speaking in a manner and using language that is not conducive to civil discourse."

Over and over, Republicans turned back Democratic amendments to subpoena documents from the White House, State Department, Defense Department and budget office. By the same 53-47 party-line, they turned away witnesses with front-row seats to Trump's actions including acting White House chief of staff Mulvaney and Bolton, the former national security adviser critical of the Ukraine policy.

Only on one amendment, to allow more time to file motions, did a single Republican, Sen. Susan Collins of Maine, join Democrats. But it, too, was rejected, 52-48.

Motions from the Trump legal team were due Wednesday morning, but the attorneys weren't filing any, said Jay Sekulow, one of the president's lawyers. That means there will be no motion to dismiss the case as some Republicans had discussed.

As the visitors' gallery filled earlier with guests, actress-and-activist Alyssa Milano among them, and Trump's most ardent House allies lining the back rows, the day that began as a debate over rules quickly took on the cadence of a trial proceeding over whether the president's actions toward Ukraine warranted removal from office.

"It's not our job to make it easy for you," Rep. Adam Schiff, the chairman of the House Intelligence Committee leading the prosecution, told the Senate. "Our job is to make it hard to deprive the American people of a fair trial."

White House counsel Pat Cipollone, the president's lead lawyer, called the trial "a farce." He scoffed that the House charges against Trump were "ridiculous."

The White House legal team did not dispute Trump's actions, when he called Ukraine and asked for a "favor," which was to investigate Biden as he withheld military aid the ally desperately needed as it faced off with hostile Russia on its border. But the lawyers insisted the president did nothing wrong.

"Absolutely no case," Cipollone said.

Schiff, the California Democrat, said America's Founders added the remedy of impeachment in the Constitution with "precisely this type of conduct in mind — conduct that abuses the power of office for a personal benefit, that undermines our national security, and that invites foreign interference in the democratic process of an election."

Said Schiff: "It is the trifecta of constitutional misconduct justifying impeachment.''

Sekulow, the other lead lawyer on Trump's team, retorted, "I'll give you a trifecta," outlining complaints over the House Democrats' impeachment inquiry process.

Democratic Rep. Adam Schiff, one of the House impeachment managers, argued for an amendment offered by Senate Minority Leader Chuck Schumer to subpoena documents from the Defense Department during President Donald Trump's impeachment trial Tuesday.

In Davos, Trump repeated his attacks on Democratic House managers serving as prosecutors in the trial, saying that he'd like to "sit right in the front row and stare at their corrupt faces" on the Senate floor during the trial but that his attorneys might have a problem with it.

And he said he wants to deliver the State of the Union as scheduled on Feb. 4 even if the trial is ongoing, calling the address "very important to what I am doing" in setting his administration's agenda.

The impeachment trial is set against the backdrop of the 2020 election. All four senators who are Democratic presidential candidates were off the campaign trail, seated as jurors.

"My focus is going to be on impeachment," Sen. Bernie Sanders, the Vermont independent, told reporters.

McConnell stunned senators and delayed the start of proceedings with his decision to back off some of his proposed rules. He made the adjustment after encountering resistance from Republicans during a closed-door lunch meeting. Senators worried about the political optics of "dark of night" sessions that could come from cramming the 24 hours of opening arguments from each side into just two days.

Collins and Sen. Lisa Murkowksi of Alaska, who often buck party leadership, along with a substantial number of other Republicans, wanted to make the changes, according to people familiar with the situation.

It was only when the clerk started reading the dry language of the resolution that the hand-written changes to extend debate to three days became apparent. It also allowed the House impeachment record to be included in the Senate.

The turnaround was a swift lesson as White House wishes run into the reality of the Senate. The White House wanted a session kept to a shorter period to both expedite the trial and shift more of the proceedings into late night, according to a person familiar with the matter but unauthorized to discuss it in public.

White House adviser and former Florida Attorney General Pam Bondi argues against an amendment offered by Senate Minority Leader Chuck Schumer on Tuesday.

Trump's legal team, absent its TV-showcase attorneys, Alan Dershowitz and Kenneth Starr who were not in the chamber, argued that in seeking new evidence the House was bringing a half-baked case.

But Rep. Zoe Lofgren of California, one of the House managers and the first woman to argue for the prosecution in a presidential impeachment trial, said the House wasn't asking the Senate to do the job for them. "The House is asking the Senate to do its job, to have a trial," she said. "Have you ever heard of a trial without evidence?''

The White House had instructed officials not to testify in the House inquiry, and refused to turn over witnesses or documents, citing what is says is precedence in defiance of congressional subpoenas.

Democrat Schiff displayed video of Trump himself suggesting there should be more witnesses testifying.

One by one, the House managers made the case, drawing on their own life experiences.

Rep. Val Demings, D-Fla., a former police chief, said she never saw anyone take "such extreme steps to hide evidence.'' Rep. Jason Crow, a former Army Ranger who served in Iraq and Afghanistan, seemed to capture senators' attention when he told them near he knew the hour was late, but it was morning in Ukraine where soldiers were waking up to fight Russia, depending on U.S. aid.

It was when Rep. Jerrold Nadler, the House Judiciary Committee chairman also leading the prosecution, said the White House lawyers "lie" that Cipollone and Sekulow retorted that Nadler should be embarrassed and apologize, leading to Roberts' admonition.

No president has ever been removed from office. With its 53-47 Republican majority, the Senate is not expected to mount the two-thirds vote needed for conviction.