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:

Senate Republicans introduced a big twist, then seemed to change their mind

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 he had enough votes to advance it, after meeting privately with moderate and vulnerable Republicans like 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 earlier this month.

Then late Monday, on the eve of the Senate’s consideration of the trial rules, McConnell, R-Ky., released trial rules that included a big 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.

But with no explanation, McConnell appeared to change his mind. He took the Senate floor with a new resolution, allowing the 24 hours of arguments on each side to play out across three days instead of two. That means each side would be able to wrap up by bedtime.

It’s a cover-up! Democrats’ objections echo House Republicans’ arguments last fall

Last fall, House Republicans blasted the Democratic-led impeachment inquiry as a cover-up after House Democrats opted for closed-door depositions from witnesses. Democrats eventually called the witnesses to testify in open hearings and defended the process as similar to past impeachment inquiries in which evidence is initially collected behind closed doors before being made public.

Republicans though were quick to brand the inquiry “Soviet-style,” taking place in a “super-secret bunker” and comparing Democrats to an “angry pack of rabid hyenas.”

“I would say if we pulled this stunt — you’d be eating us alive,” Sen. Lindsey Graham, R-S.C., told reporters last October.

Then came McConnell’s plan to hit fast-forward on the Senate trial.

While McConnell later changed the rules to allow three full days for each side to make their arguments, Democrats were quick to jump, decrying a trial held in the “dead of the night” and calling the trial “rigged” – echoing the same arguments House Republicans last fall.

“If the president is so confident in his case, then why won’t he present it in daylight?” Senate Democratic Leader Chuck Schumer said at a press conference Tuesday.

Rep. Adam Schiff, the Democratic chair of the House Intelligence Committee, who is outlining the charges against the president, acknowledged the change in his opening statements on the Senate floor and reminded senators that regardless of timing they must still act as impartial jurors, even if voters don’t expect it.

Americans “believe the result is pre-cooked,” said Schiff. “The president will be acquitted — not because he is innocent. He is not. But because the senators will vote by party, and he has the votes.”

The question of witnesses remains unresolved

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 the process could take months.

Now adding intrigue to the upcoming impeachment trial is whether the Senate will vote to subpoena witnesses or documents, including Trump’s former national security adviser John Bolton. While Bolton was never subpoenaed by the House, he has said he would be willing to testify before the Senate.

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.

Under McConnell’s proposed rules, witnesses are a possibility but not guaranteed. After each side has a chance to make their case, the senators can ask questions for up to 16 hours followed by another four hours of arguments. Then the Senate could decide whether to subpoena witnesses or documents.

“No testimony shall be admissible in the Senate unless the parties have had an opportunity to depose such witnesses,” the rules state.

Only after that will the Senate cast its 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 producer Trish Turner contributed to this report.