The United States Senate opened the first impeachment trial of a president in 131 years yesterday, starting on a solemn note with the formal reading of charges against William Jefferson Clinton but soon grinding to a halt amid deep disagreements over what to do next.

Chief Justice William H. Rehnquist entered the majestic Senate chamber at 1:19 p.m. to take over as presiding officer for the duration and swear in the jury of 100 grim-faced senators, who then marched to the well one by one to sign an oath book pledging to do “impartial justice.”

Yet even as the trial formally began, the rules for how it would be conducted, how long it would last and how it would end remained unwritten – uncertainties at the center of a struggle that threatened to plunge the Senate into the same partisanship that consumed the House during its impeachment debate last month. After recessing without issuing the required summons to the president, Senate leaders engaged in an afternoon of up-and-down negotiations ending with signs that a resolution could come today.

“We may be closer than we think we are,” Senate Majority Leader Trent Lott (R-Miss.) said at a collegial early evening news conference with Minority Leader Thomas A. Daschle (D-S.D.) intended to demonstrate that talks were back on track.

In a final bid to bridge party lines, the Republican and Democratic caucuses will hold a rare, if not unprecedented, joint meeting at 9:30 a.m. today, essentially a closed session of the entire Senate. However, even if it cannot reconcile differences over calling witnesses, the Senate intends to reconvene at 1 p.m. to approve a trial schedule with opening arguments next week.

By last night, the parties were circulating competing plans. The Democratic version envisioned seven days of arguments by prosecutors and defense lawyers beginning Tuesday, with the Senate voting on the articles of impeachment Jan. 26. The Republican version would start presentations Wednesday, limiting each side to 24 hours of arguments, followed by motions and then votes on subpoenas for witnesses. An unspecified amount of time would be allowed for testimony, followed by eight hours of closing arguments and votes in early February.

The confusion on Capitol Hill underscored the extraordinary nature of the constitutional confrontation now engaging all three branches of government. The only other time the Senate has considered removing a president from office came during the 1868 trial of Andrew Johnson, in an era when there were no independent counsels, no DNA evidence, no television cameras and no direct election of senators.

Johnson was acquitted by a single vote after a 2½-month trial on charges that he illegally fired a dissident Cabinet officer without congressional permission, the climax of years of political battling over his Reconstruction policies easing the return of Confederate states to the Union.

Clinton went on trial yesterday for “high crimes and misdemeanors” of a far different stripe, accused in two articles of impeachment of perjury and obstruction of justice stemming from his efforts to keep secret his affair with former White House intern Monica S. Lewinsky. Under the Constitution, conviction requires a two-thirds vote, or 67 senators if all are present.

As he has through much of the year-long scandal, Clinton did his best to ignore the political crisis, at least publicly. Aides said he remained in the White House residence during the morning Senate proceedings and did not watch, then had his weekly luncheon with Vice President Gore during the afternoon session.

In his only public appearance of the day, the president continued his business-as-usual approach by announcing more money for after-school programs during an East Room ceremony that included no mention of impeachment. Still, to some in the room he appeared distracted at times, twice noting the youth of Rep. Harold Ford Jr. (D-Tenn.), “a 28-year-old, young guy with his whole life before him,” as Clinton observed wistfully.

His advisers were not so quiescent, lashing out after weeks of deference to the Senate. White House press secretary Joe Lockhart complained that starting arguments without a decision on witnesses would be “manifestly unfair to the president” and warned that allowing witnesses as demanded by House prosecutors would force a lengthy delay.

“Whether you’re in a trial in the Senate or you’re in traffic court, the idea that when you go and start a proceeding that the rules and procedures . . . are not clear to you at the beginning of the process is not fair,” Lockhart said. “You cannot have a process . . . where the rules get made up as you go.”

With the Johnson trial as its only real guide, the Senate plowed forward with the formalities conscious of the history it was following and making. The House team of 13 prosecutors, or “managers,” led by Judiciary Committee Chairman Henry J. Hyde (R-Ill.), filed across the Capitol to present the charges at 10 a.m. Speaking in a low, husky voice, Hyde read from the two articles asserting that Clinton “has brought disrepute on the presidency [and] has betrayed his trust as president.”

The Senate returned at 1 p.m. to officially begin considering the articles. Rehnquist, wearing his black judicial robe with gold braids on the sleeves, was escorted into the hushed chamber by a bipartisan delegation of six senators and ascended the dais to be sworn in.

As Senate president, Gore could have performed the task but opted not to, leaving it to Senate president pro tempore Strom Thurmond (R-S.C.). The hulking chief justice towered over the 96-year-old senator, who was hunched over as he read the oath.

Rehnquist then administered to the senators the same oath he had just taken. The new jurors stood at their desks with their right hands raised as Rehnquist asked: "Do you solemnly swear that in all things appertaining to the trial of the impeachment of William Jefferson Clinton, president of the United States, now pending, you will do impartial justice according to the Constitution and laws, so help you God?"

To complete the act, the senators were called to the well in alphabetical order to sign the oath book; each was given a black-ink Parker Vector pen inscribed “United States Senator” to inscribe his or her name and then keep as a memento.

For many senators, it was a moment to come to terms with a trial many of them never expected. Some later described wondering how it all came to this; others were struck by the weight of their responsibility. All the more troubling for some was the party-line schism over how to proceed.

“I am really sick at heart to think that we cannot act like jurors,” said Sen. Ted Stevens (R-Alaska). “We have got to stop acting like partisan senators and act like jurors.”

Under the rules, as soon as they were sworn in, the senators were supposed to issue a writ of summons to the president reciting the charges and advising him of the deadline for his response, but the partisan rift forced the Senate to recess instead.

The rest of the day was absorbed in fitful talks. After an impromptu gathering of 30 to 40 senators from both parties on the Senate floor following Hyde’s presentation, Lott decided to convene a bipartisan meeting of all senators. But as the parties caucused separately shortly before Rehnquist arrived, the train seemed to veer off the tracks and the meeting was scrubbed, prompting partisan recriminations that both sides had been straining to avoid.

With no agreement, Lott then decided to call the Senate back and force votes on the alternate plans at 5 p.m. But at the last minute, he backed off after he and Daschle agreed to try one more time to find accommodation by rescheduling the joint caucus meeting for this morning in the ornate Old Senate Chamber.

Appearing together before reporters at 5:30 p.m., Lott and Daschle acknowledged the day’s topsy-turvy developments. “We think that the best way to keep calm and cool and dignified is to look at each other and talk to each other,” Lott said. Reflecting the difficulties of reaching agreement among lawmakers who jealously guard their prerogatives, Lott added: “Senator Daschle and I are not dictators. . . . We have to bring along 98 other senators. We are struggling very hard to do that. We ask for the indulgence of the American people.”

The Senate’s fickle behavior appeared to stem from miscommunications, miscalculations and the inherent conflict between partisan interests and the desire to keep the trial from appearing overly partisan.

Republicans accused the Democrats of spurning the bipartisan caucus, while Democrats said Lott never directly proposed it to Daschle. That cross-talk led directly to the decision to go forward with a showdown vote. But then that decision was reversed as senators came to recognize that their plans had not been completely drafted and came to fear heading down the same road as the House.

The situation was further complicated by the multilateral negotiations, with senators exchanging proposals and counter-proposals with House prosecutors as well as each other. The only thing that was clear was that last week’s hopes of a quick five-day trial have vanished as every viable alternative now envisions at least two weeks of proceedings.

The main difference in the plans promoted by the two parties last night centered on whether to accede to House prosecution demands for 10 to 15 witnesses. Senate Democrats adamantly oppose live testimony and the White House offered Wednesday night to rely entirely on the evidence collected by independent counsel Kenneth W. Starr and forgo cross-examinations if the Senate went forward without witnesses.

But the House prosecutors maintained witnesses were essential to assess clashing accounts from different players. “The only way to resolve that conflict is to look at their demeanor, evaluate their credibility,” said Rep. Asa Hutchinson (R-Ark.), one of the House managers.

In addition to Lewinsky, the prosecution team wants to call presidential confidant Vernon E. Jordan Jr. and Oval Office secretary Betty Currie, as well as a variety of Clinton advisers, including White House Chief of Staff John D. Podesta and his predecessor, Erskine B. Bowles, former deputy chief of staff Harold M. Ickes, communications strategist Sidney Blumenthal and former campaign consultant Dick Morris.

Democrats argued that there is little that witnesses could add to the voluminous record and that even one witness would open the floodgates to a tortuous process that could last months. To avoid that, their plan called for the cases to be presented without testimony beginning Tuesday, allotting two days for opening arguments by each side, followed by two days of questions and another day of closing arguments.

Under the Republican plan, both sides would be allowed to start with motions, which would be due Monday and considered Wednesday. The White House could move to adjourn or dismiss the case outright, which would require a 51-vote majority. If such motions failed, presentations would then commence, with each side granted 24 hours over as many days as that takes.

After that stage, the Senate would again consider motions and vote on subpoenas for witnesses requested by either side, one at a time. After all testimony is heard, senators would hear closing arguments. The written plan mentioned no end date, but GOP senators hope to vote between Feb. 5 and 12.

Staff writer Juliet Eilperin contributed to this report.