The ongoing battle to compel former White House counsel Don McGahn's testimony has opened a new front on the war over President Donald Trump's impeachment as the articles appear stalled in making their way to the Senate until at least early January.

Dueling court briefs over a House subpoena for McGahn's testimony have hardened the battle lines on an already divisive impeachment. Democrats' counsel argues that the investigations into Trump are ongoing even after last week's historic vote and any new information from McGahn could potentially lead to drafting additional articles of impeachment. The Justice Department, meanwhile, says Democrats squandered their opportunity to expedite the case by voting on impeachment articles that effectively ended their investigation and wants the lawsuit dismissed altogether.

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The resulting legal decision carries major implications for the evidence allowed to be presented at Trump's eventual Senate impeachment trial.

In filings this week on behalf of the House Judiciary Committee, Democratic counsel Douglas Letter wrote to the D.C. Circuit Court of Appeals that there's still an "urgent need" for the former White House official's testimony that could be helpful in the Senate's impeachment trial. He also raised the prospect of more articles, though that hasn't been publicly discussed by Democratic lawmakers.

"McGahn's testimony would thus inform the House's decision-making about impeachment and presentation of the articles in a Senate trial," Letter wrote in his filing. "McGahn's testimony is also relevant to the committee's ongoing investigations into presidential misconduct and consideration of whether to recommend additional articles of impeachment."

Meanwhile, the Justice Department, which is representing McGahn and the White House in the case, argues that there's no longer an urgency to resolve the lawsuit given that Trump has been impeached. DOJ wrote in its filing that the appeals court should "dismiss this fraught suit between the political branches for lack of jurisdiction."

"If this Court now were to resolve the merits question in this case, it would appear to be weighing in on a contested issue in any impeachment trial," DOJ lawyers wrote. "The now very real possibility of this Court appearing to weigh in on an article of impeachment at a time when political tensions are at their highest levels – before, during, or after a Senate trial regarding the removal of a President – puts in stark relief why this sort of interbranch dispute is not one that has 'traditionally thought to be capable of resolution through the judicial process.'"

Trump was impeached last week for an abuse of power and obstruction of Congress related to his dealings with Ukraine. Democrats have recommended his removal from office over allegations that he solicited Ukraine to intervene in the 2020 election by opening political investigations that could benefit his reelection and that he used military aid and a White House meeting as leverage.

House Democrats decided against drafting an obstruction of justice article that would have likely included evidence uncovered by former special counsel Robert Mueller's investigation into Russian election interference in 2016 and whether Trump sought to obstruct the probe. McGahn was a key witness in Mueller's investigation into potential obstruction of justice.

Still, no Democratic members have floated impeaching Trump for a second time. But the arguments in the McGahn appeals case show Democrats' flexing their muscles and seeking to bolster their case on impeachment, especially before the articles make their way to the GOP-led Senate.

A federal judge in D.C. ruled last month that McGahn is required to appear before the House Judiciary Committee and that Trump cannot assert "absolute immunity" to prevent his testimony. Democrats' lawsuit was prompted by McGahn skipping his May hearing for which he was subpoenaed to testify before the Judiciary Committee. But the ruling was appealed by DOJ and arguments before the appeals court are scheduled for Jan. 3.

In light of the McGahn court filings, Trump tweeted quotes from conservative media figures who panned Democrats for continuing to focus on the Mueller investigation despite keeping them out of the two articles of impeachment. Republicans, in alignment with the White House, have condemned Democrats for not giving Trump and his legal team a fair chance to defend the president, though he was given some due process rights during the hearings before the Judiciary Committee. They've also criticized Democrats for what they characterize as a rushed process that resulted in a weak case, and they've argued that it's not the job of the Senate to gather evidence that House Democrats chose not to wait for by declining to go to court to compel testimony from key officials like former national security adviser John Bolton.

"The Radical Left, Do Nothing Democrats have gone CRAZY. They want to make it as hard as possible for me to properly run our Country!" Trump tweeted Tuesday.

The fight over McGahn comes amid a recent release of emails from the Trump administration, prompted by a Freedom of Information Act request from the Center for Public Integrity, that there was a discussion about withholding Ukraine's security assistance not long after the July 25 call in which Trump asked Ukrainian President Volodymyr Zelenskiy to "do us a favor."

New emails show that 90 minutes after the July 25 call that ultimately sparked the impeachment process, the White House budget office directed the Pentagon to freeze the nearly $400 million in congressionally approved military aid. Other emails found that budget staffer Michael Duffey told the Defense Department to keep the decision under wraps because of the "sensitive nature of the request."

Office of Management and Budget spokeswoman Rachel Semmel told The New York Times in a statement that "it's reckless to tie the hold of funds to the phone call." She pointed out that the announcement about the aid had happened at an earlier meeting on July 18.

Senate Democrats are pushing for Duffey to testify as a witness in the Senate's upcoming impeachment trial. Senate Minority Leader Chuck Schumer of New York has requested that Duffey and three other past and present White House officials testify because they have "direct knowledge" of the temporary freeze on Ukraine's military aid as well as the release of documents related to Ukraine. And after the release of the emails, Schumer renewed his call for Duffey and others – Bolton and acting White House chief of staff Mick Mulvaney – to appear.

"The email from Michael Duffey shows why it is so important for the White House to produce the requested documents and let witnesses testify under oath," Schumer tweeted Monday.

But Senate Majority Leader Mitch McConnell of Kentucky, who has vowed "total coordination" with the White House on the framework of the trial, has already dismissed his Democratic counterpart's request for witness testimony. Republican leadership appears to want an abbreviated trial so they can quickly acquit the president and move on. And even if House Democrats win and the McGahn case continues to be expedited, the party would still face obstacles related to the Senate trial given McConnell's initial resistance to witness testimony.

The impending Senate trial continues to be stalled as lawmakers are home for the holidays and don't return to Congress until the week of Jan. 6. House Speaker Nancy Pelosi of California hasn't transmitted the articles of impeachment to the Senate because she says she's holding off on naming House impeachment managers until the Senate agrees on the format of a trial.