Technically, arguments in the impeachment trial of President Trump haven’t even started. Democrats and Republicans spent Tuesday, the first full day of Senate proceedings, doing battle over the rules that will govern the trial that is set to unfold in the coming days.

That rule package was ultimately approved, on party lines, early Wednesday morning after a marathon session of debate and votes on 11 amendments to the package proposed by Democrats.

The day’s significance, however, extends beyond the realm of parliamentary minutiae and dry process detail.

The way that senators crafted and passed the rules—and the way those rules were received by the House Democrats prosecuting the case against Trump and the lawyers defending him—revealed some fundamental truths about this historic trial that could hold up until the final vote to acquit or remove the president.

The most unexpected thing that became clear on Tuesday—and possibly the most significant—was the emergence of some semblance of boundaries for the most powerful person in this process: Majority Leader Mitch McConnell. The Kentucky Republican, whose reputation for political maneuvering is almost mythical at this point, came into the trial phase of the impeachment process touting an extraordinary degree of unity within his conference of 53 senators.

As the Senate prepared to receive articles of impeachment from the House last week, McConnell declared that he had within that group the 51 votes required to pass a rules package—giving him the ability to bypass Senate Democrats.

On Monday night, McConnell released a rules framework. As expected, it stipulated a vote on subpoenaing additional witnesses and documents after each side’s arguments concluded. More controversially, it provided for up to 24 hours of argument to be used by each side—but those hours could only be used over a period of two days for each side. Additionally, the rules did not automatically admit the evidence collected by the House during its impeachment inquiry.

In the ensuing hours, GOP senators raised concerns about the structure of the trial, forcing McConnell to back down. The resolution he ultimately introduced added an extra day for each side to use their time and admitted House evidence—changes that only became known when the text of the rules was read aloud on the Senate floor Tuesday morning.

The usual suspects—like Sen. Susan Collins (R-ME)—pushed behind the scenes for these changes. But other Republicans who are hardly considered swing votes in the trial also indicated there was a wider backlash against the rules as proposed.

Sen. Mike Braun (R-IN) told reporters, “There was pretty broad agreement on the idea that when it came to any evidence that’s already been heard in the House, that just gets accepted into the record.”

A close McConnell ally, Sen. Roy Blunt (R-MO), echoed that point. “Easier to accept everything you already saw and we already saw than to have a fight about whether there was evidence or not, or whether there had already been a lot of witnesses,” he told reporters Tuesday night.

Blunt also said he ultimately didn’t see a problem in giving each side an extra day to make its case. Indeed, that change privately came as a relief to members on both sides who would be spared the long nights in the Senate chamber they would have faced if they had kept to the breakneck trial pace initially laid out.

“The issues of the day,” said Blunt, “were reflected in the changes to the rules.”

Some in the Capitol wondered if the apparent balk from McConnell, ever the tactician, wasn’t part of some deeper strategy. But some Democrats urged a simpler explanation.

“It tells me,” said Sen. Chris Coons (D-DE), “McConnell doesn’t have this completely nailed down in his caucus.”

If the rules change illustrated possible limits to McConnell’s authority, the votes that took place immediately afterward were a bitter chaser to any lingering hopes that a bloc of Republicans might consistently push the GOP leader on other matters.

As he has promised for weeks, Minority Leader Chuck Schumer (D-NY) used his power to force a lengthy series of votes on subpoenaing documents and testimony withheld by the Trump administration. Several GOP senators have publicly said they are open to hearing new evidence in the trial—and others, like Collins and Sen. Mitt Romney (UT), have indicated they are likely to vote in favor of new evidence.

But not a single Republican voted in favor of obtaining that new evidence at the onset of the trial. McConnell has made clear for weeks that he’d prefer to hold that vote after arguments in the case are heard—and on that point, all Republicans have been in lockstep with their leader.

Even if they were all voted down on party lines, the content of the amendments Schumer offered to the rules package revealed important details about how the Democratic side will approach the trial. While much of the debate about new evidence has centered on witnesses like acting White House Chief of Staff Mick Mulvaney and former National Security Adviser John Bolton, Senate Democrats charged into Tuesday with a renewed focus on the documents that could tell the story of Trump World’s scheme to pressure Ukraine for political favors.

To that point, the first three amendments Schumer offered were all to issue subpoenas for documents withheld by the White House, the State Department, and the Office of Management and Budget. It wasn’t until the evening that an amendment was offered to subpoena Mulvaney. After that, Democrats returned to documents with an amendment to subpoena materials from the Pentagon.

The tactic made for a late night on Capitol Hill. But Democrats considered it a sharp strategy. Republicans have dangled the idea of witness “reciprocity”—calling someone like Hunter Biden, for example, in exchange for Bolton—in order to complicate Democrats’ calls for evidence. But such a tactic will be harder to pull off for documents, which Schumer and others believe could be as explosive as new witnesses in demonstrating Trump’s culpability.

The lengthy rules debate also served as something of a mini-preview of the trial arguments, with each side—particularly the Democrats—using debate about amendments to explore the issues at the heart of the trial. The team of seven impeachment managers, led by Rep. Adam Schiff (D-CA), went through the factual timeline of the Ukraine allegations, referred to witness depositions, and played video clips of Trump’s own words to prove their points.

That seemed to wear on some Republicans, who grumbled that Schiff and company were too eager to take advantage of initial interest in the trial to lay out their case.

“They may very well think, with this case and this repetition, this is the last time that the general public will pay any attention and they’d better drive this home as dramatically as they can,” said Blunt.

The public has seen plenty of the Democrats’ impeachment team—but Trump’s defense lawyers were basically an unknown quantity until the wall-to-wall TV coverage of Tuesday’s proceedings. Those lawyers burst into the public eye with the strident, combative approach embodied by their client. “The president has done absolutely nothing wrong,” said White House counsel Pat Cipollone.