As a result, the case has continued to evolve even weeks after the House’s impeachment proceedings formally concluded.

Former national security adviser John Bolton’s surprise offer Monday to testify at Trump’s impeachment trial — after refusing the House’s request to appear late last year — was the exclamation point. But it was only the latest in a string of revelations and promises of new information.

Last week, a federal judge authorized Lev Parnas, the indicted associate of Trump’s personal lawyer Rudy Giuliani, to share documents and contents of a seized iPhone with House investigators. In recent days, documents emerged shedding new light on Trump’s order to temporarily withhold military aid from Ukraine.

And last month, House Democrats revealed that an aide to Vice President Mike Pence, Jennifer Williams, had submitted a classified supplement to her deposition, which the House Intelligence Committee is seeking permission to release publicly. Williams had previously called Trump’s efforts to pressure Ukraine to investigate his political rivals “unusual and inappropriate.”

The new revelations in the Ukraine scandal present precarious political questions as House Democrats and Senate Republicans jockey for a strategic edge in the upcoming trial. But it also underscores that the case itself is evolving — and will likely continue to evolve — regardless of what happens in the Senate trial. That reality presents risks as new and potentially crucial evidence is layered on top of the House’s findings.

In a nod to that uncertainty, Senate Democrats have demanded that the trial include relevant testimony from witnesses — including Bolton and current senior White House officials — in addition to the production of documents that the State Department, Pentagon and White House budget office have refused to provide.

“What are you hiding, President Trump? What are you afraid of, President Trump? If you think that you’ve done nothing wrong, you wouldn’t mind having witnesses,” Senate Minority Leader Chuck Schumer (D-N.Y.) said this week.

But Senate Majority Leader Mitch McConnell is brushing aside such comments. The Kentucky Republican said Tuesday he has the necessary GOP votes to move forward with a trial governed by the parameters of the 1999 proceedings against Bill Clinton, in which the decisions about whether to call witnesses or procure documents were deferred until after opening arguments. McConnell and his allies have swiped at Democrats, contending their demand for new evidence is an indication that the House failed to build a persuasive argument for impeaching Trump.

“This just shows how desperate the Democrats are, and how poorly a job they did in the House in establishing a credible case,” Sen. John Barrasso of Wyomin, a member of Senate GOP leadership, said in response to questions about whether Bolton should be called to testify. “They should have put the case together in the House. They had an opportunity to do it. And they didn’t.”

Indeed, the House impeached Trump without any of this evidence, arguing that the case was too urgent, and irrefutable, to wait. Trump, Democrats said, was threatening the integrity of the 2020 election, and the evidence they had already obtained was overwhelming. The House voted on Dec. 18 to impeach Trump, almost entirely along party lines.

The potential flood of new evidence has led some Senate Republicans to argue that the upper chamber should simply ignore it and only consider the House’s work product during the trial, whether the new evidence is damning or exculpatory. Republicans say Democrats’ self-imposed deadline to impeach Trump was arbitrary and that it is not the Senate’s job to plug holes in the House’s investigation.

“They could have taken any of these subpoenas to court. They could have spent the time, as they have in the past, to bring the witnesses forward,” Sen. Thom Tillis (R-N.C.) said. “That’s the way the process works. That’s the way it needs to work.”

House Intelligence Chairman Adam Schiff (D-Calif.) did not rule out the possibility of subpoenaing Bolton on Tuesday, but said the Senate trial was the most appropriate venue for his testimony.