Senators will almost certainly get to vote on whether or not to call impeachment witnesses. The resolution laying out the rules of the trial, which will be presented Tuesday, is expected to mandate that senators can take up-or-down votes on calling for witnesses and documents.

Yes, but: Those votes won't come until the House impeachment managers and President Trump's defense team deliver their opening arguments and field Senators' questions.

Some moderate Senate Republicans have pushed for this precise language, including Sen. Susan Collins, who faces a tough re-election. The language is "critical" for her, a source familiar with Collins' thinking told Axios.

What to watch: Senate Minority Leader Chuck Schumer is also expected to use the vote on the resolution to push Democrats' messaging, with an eye trained on weakening the GOP majority in the Senate and clawing back the more vulnerable seats in November.

A Democratic leadership aide told Alayna that they will force votes on subpoenaing key witnesses such as Trump's former national security advisor John Bolton, acting Chief of Staff Mick Mulvaney, Mulvaney's senior advisor Robert Blair, and top White House budget official Michael Duffey, as well as relevant documents.

This has concerned some vulnerable Senate Republicans. "The Democratic amendments that will be offered in the beginning will be designed to screw us," a Republican Senate aide told Axios. "Like, 'How can we cut these to look like an ad?'"

"Democrats are going to try and amend these things until the cows come home, but remember, senators can't talk [during the trial]," a GOP leadership aide said. "So there won't be a clip of Susan Collins voting no on this."

Reality check: Although senators are required to remain silent during the trial, all votes will be recorded.

Although senators are required to remain silent during the trial, all votes will be recorded. "This is not the political silver bullet that Democrats think it will be," the aide added. "Remember that every time Schumer offers up something to tweak our organizing resolution, it's an implicit way of saying that the House investigation came up short."

Meanwhile, a key strategy within the Trump team's defense mechanism is the notion that the articles themselves are not criminal, and therefore are not impeachable offenses, even if proven.

In turn, they'll claim this is why the debate over witnesses is bogus.

"If a person is indicted on something that is not a crime, you don't call the witnesses," Alan Dershowitz, a member of Trump's defense team, said earlier today on CNN's "State of the Union."

Expect to hear this line again.

Go deeper: Inside Trump's strategy for blocking impeachment witnesses