A Government Accountability Office report accusing President Trump of violating the law is taking on heightened importance at the start of his impeachment trial.

Why it matters: The key argument Trump's legal team plans to make is that the articles of impeachment are deficient on their face because, unlike previous impeachment attempts, they don’t allege that Trump broke the law.

But Democrats plan to highlight how the report from the nonpartisan government agency frequently referred to as the "congressional watchdog“ clearly finds that the Trump administration violated the law.

“This is a big part of our case,” a Democratic leadership aide told Axios. “It shows the extent to the president went to advance his scheme; he went so far to break the law. It's an important piece of evidence and only adds to the mountain and body of evidence that we already have."

What to watch: Whether new evidence uncovered by the House will be blocked from the Senate trial. The organizing resolution laying out the terms for the impeachment trial, released by Senate Majority Leader Mitch McConnell last night, left open the chance that the Senate could decline to review such evidence.

What we're hearing: Sources working with the president’s legal team told reporters Monday that the impeachment of Trump is illegitimate and historically aberrant because it doesn’t accuse Trump of committing a crime.

One said: "We’re going to trial on a specific set of charges, spelled out in the charging document. The charging document doesn’t include anything about that GAO report. So in our view that’s not properly part of the accusation that‘s been brought to the Senate.”

Meanwhile, a Democratic aide working on impeachment said: "The president was so eager and so determined to turn the screws and ratchet up the pressure on Ukraine to do his political dirty work, that he was willing to break the law by withholding much needed security assistance from Ukraine.

“That’s the reason why they don't want that document” to be considered, the aide said.

The backdrop: The GAO found that the White House Office of Management and Budget violated the law when it withheld military aid to Ukraine.

The agency's report, which dropped just hours the Senate impeachment trial began last Thursday, determined that the OMB violated the 1974 Impoundment Control Act, which mandates that the White House and its agencies release funds appropriated by Congress.

Yes, but: The GAO's only tool to hold violators of the ICA accountable is to sue the administration to distribute the funds. OMB already released the aid to Ukraine in September.