Partisanship gripped Congress on Tuesday as Senate Republicans and Democrats clashed on the rules governing the impeachment trial of President Donald Trump.

Here are three things to know:

McConnell proposed a twist to the rules, then backed down

Earlier this month, Senate Majority Leader Mitch McConnell promised he would put forward rules “very similar” to those used during President Bill Clinton’s impeachment trial — pushed when Democrats were in control. He also announced that he had enough votes to advance it, after meeting privately with moderate and vulnerable Republicans, such as Maine Sen. Susan Collins.

“All we’re doing here is saying we’re going to get started in exactly the same way that 100 senators agreed to 20 years ago,” McConnell said at the time.

Then, on the eve of the Senate’s consideration of the trial rules late Monday, McConnell, R-Ky., revealed a twist. Each side would be given 24 hours to present their arguments, as they were in 1999. But unlike 20 years ago, there would be a stricter time cap: The 24 hours must take place “over up to 2 session days,” according to the rules pitched late Monday.

That rule would have meant that because of the 1 p.m. planned kickoff time each day, Senate arguments would have bled into the early morning hours of the next day while much of America was asleep.

Then without explanation Tuesday, McConnell announced a new plan: arguments on each side could play out across three days instead of two. That means each side would be able to wrap up by bedtime.

McConnell also agreed to have evidence from the House inquiry automatically admitted as evidence in the Senate trial.

According to congressional officials, some Republicans — including Maine’s Collins — had pushed back behind closed doors. Collins’ office said that she and others expressed concerns that the compressed timeline didn’t follow the Clinton precedent closely enough.

Sen. Mike Braun, R-Ind., said it wasn’t just Collins and that there were “pretty broad reservations” across the caucus on the initial rules.

Added Wisconsin Republican Sen. Ron Johnson, “We are not trying to hide testimony in the wee hours of the morning.”

Democratic amendments to demand witnesses initially failed

The White House refused to comply with congressional subpoenas during the House inquiry, blocking key witnesses from testifying and refusing to supply key documents. House Speaker Nancy Pelosi said she didn’t want to wait for the courts to weigh in because that process could take months.

On Tuesday, Democrats tried again to force the issue with amendments to the trial rules that would have guaranteed the Senate would hear from specific witnesses or obtain documents. Those amendments were failing along party lines, as the Senate continued to debate late Tuesday.

Sen. Mazie Hirono of Hawaii said Democrats have “numerous” additional amendments to file, even if they lack the votes, because they think a debate on witnesses and documents is an important one to make.

But the strategy has its limits. Sen. Dianne Feinstein, D-Calif., told reporters, “I don’t think there is a point If we lose every one. So hopefully one or two will be it and we’ll stop.”

Whether John Bolton or others will testify remains in question

While Republicans defeated Democratic amendments on witnesses and documents on Tuesday, that’s not the final word on the subject.

In a statement issued to reporters, Collins said she remains open to the idea after opening arguments and the first round of questions by senators. That would be similar to the approach used in Clinton’s 1999 trial.

“It is likely that I would support a motion to subpoena witnesses at that point in the trial just as I did in 1999,” she said.

One big question remains whether the Senate will vote to subpoena Trump’s former national security adviser John Bolton.

Other witnesses have described Bolton as objecting to the pressure campaign against Ukraine, calling it a “drug deal” cooked up by Trump’s personal lawyer Rudy Giuliani. While Bolton was never subpoenaed by the House, he has said he would be willing to testify before the Senate.

Next stop though would be working through the 24 hours of testimony on either side. House impeachment managers — Reps. Adam Schiff and Zoe Lofgren, both California Democrats — and White House lawyers Pat Cipollone, Jay Sekulow and Pat Philbin began previewing their arguments.

The two sides were expected to resume arguments mid-afternoon on Wednesday. The Senate remains several days away from a final vote on whether to remove the president from office — an unlikely prospect as most Republicans remain firmly behind Trump.

“We’d rather not be doing this,” said Sen. Joni Ernst, R-Iowa.

ABC News’ Trish Turner, Benjamin Siegel and Devin Dwyer contributed to this report.