Washington (CNN) After being impeached , President Donald Trump is hoping to move quickly to a vigorous defense in the Senate and is distressed the trial he hopes will vindicate him might be delayed.

"What are they doing?" Trump asked a top Republican ally, Sen. Lindsey Graham, upon learning Thursday morning that House Democrats may withhold sending articles of impeachment to the Senate until they feel assured there will be a fair trial.

"I said, 'Mr. President, I don't know,'" Graham told reporters before traveling to the White House to discuss the matter further with Trump.

The uncertainty threw a wrench into long-laid plans by the White House to mount an effort at exoneration once the impeachment proceedings move across Capitol Hill to the upper chamber. Trump and his aides have long eyed a Senate trial as the venue for eventual vindication in the saga, viewing the Republican-led chamber as a lock to acquit the President.

One possible avenue for Trump is looking back, to Barack Obama, with a suggestion -- supported possibly with Justice Department legal opinions -- that the former president should have been impeached for blocking congressional Republicans from fully investigating the "Fast and Furious" gun-running scandal.

Trump was spending the day at the White House, with two holiday receptions listed on his public calendar. He returned to a frigid Washington late Wednesday after a bitter and disjointed "Merry Christmas" rally in Michigan where he learned of the impeachment vote from a placard held aloft by a campaign aide.

The President met in the afternoon with Rep. Jeff Van Drew , the New Jersey lawmaker who is switching parties from Democrat to Republican in opposition to Trump's impeachment.

He said during the Oval Office meeting the impeachment felt anticlimactic.

"I don't feel like I'm being impeached because it's a hoax, it's a setup. It's a horrible thing they did," Trump told reporters when asked how it feels to be the third president impeached by the House.

Trump has hailed Van Drew's switch over the past several days, and used the unanimous Republican opposition to impeachment as evidence of the party's unity. He hopes the solidarity will extend in the Senate, but the future of the impeachment case was uncertain after House Speaker Nancy Pelosi told reporters she would only transmit the articles once the outlines of the trial were clear.

Despite his public confidence, Trump has complained in private about the historic indignity of his predicament. He has steeled himself for another political battle after years of facing down opponents in scorched-earth fashion. He previewed his approach during the Michigan rally, suggesting the state's long serving Democratic congressman John Dingell was in hell

"He was at a political rally," his press secretary Stephanie Grisham explained on "Good Morning America." "I think as we all know the President is a counterpuncher."

It wasn't clear whether Trump would continue his attacks on the late congressman after his comments drew condemnation, including from the lawmaker's widow, Rep. Debbie Dingell , who said on CNN she felt "kicked in the stomach" by Trump's remark.

One person familiar with the process said the White House was waiting to see Pelosi's next move, illustrating the fluidity of the situation.

As they await clarity, White House officials are grappling with what one aide called a "central decision point" in the legal preparations: who should play what role in Trump's defense, and what the legal strategy should look like.

Aides are still debating who should present key elements of the case against impeachment. Their decision-making has been muddied by the still-uncertain parameters of the Senate trial.

Focus on legal defense or public opinion?

While White House counsel Pat Cipollone is still expected to play a central role in Trump's defense, Trump has seriously considered bringing on at least four of his fiercest House allies to lay out a minority response to Democrats' report, which could provide the President with some of the theatrics he believes he deserves in his quest to clear his name.

Aides say Trump has advocated for an aggressive self-defense that might help shift public opinion and convince more Americans that Democrats impeached him on spurious charges.

Some Trump associates have argued to the President that Cipollone is better suited for making a legal argument in a courthouse -- and not a political one, which they feel will be needed for the audience of senators on Capitol Hill and Americans watching at home.

Trump has privately expressed concern about how effective his message is being expressed on television -- at times criticizing his surrogates while questioning how effective people think Cipollone will be on camera.

Trump said in the Oval Office on Thursday that Cipollone was doing a "great job" and that it "looks" like he would act as his lead attorney in the Senate trial.

But Trump added a "couple others" would help present his defense.

Adding to the confusion is the feud Cipollone remains locked in with the acting chief of staff Mick Mulvaney. White House sources say the spat has only worsened as impeachment has taken its course. Officials familiar with the dynamic often liken it to a power struggle, though it is Cipollone who has amassed power while several others say Mulvaney's sway on the President has diminished.

Decisions about the impeachment trial team have proved difficult to make so far because the rules of the proceedings remain unclear.

As the Senate trial draws closer, Cipollone and McConnell have spoken almost daily, coordinating closely for what House Democrats already fear will be a trial tilted in favor of the President. While Trump had privately advocated for a theatrical trial that doesn't just acquit him, but vindicates him, he has begun to come around to McConnell's idea of a truncated timeline with no live testimony.

White House officials preliminarily expect that the trial could be kept as short as a week, where House managers receive just a few days to lay out their case, the White House Counsel's office -- with Cipollone presenting -- gets time to respond, and then lawmakers hear both sides' closing arguments. Outside lawyers, including the President's personal attorney Jay Sekulow, could be brought in to argue the issue surrounding the second article: obstruction of Congress.

Cipollone and the rest of the defense team are currently crafting two separate cases, according to people familiar with the plans: one to present against the article for abuse of power, and one against the article for obstruction of Congress.

On the latter charge, aides say the White House counsel's office is preparing to "dismantle" the Democrats' arguments.

"No one is losing sleep over that one," one White House source said.

Attacking Obama and Democrats

The White House is expected to rely on Justice Department legal opinions issued under Democratic administrations to make their case -- including arguing that, under House Democrats' standard, Obama should have been impeached for withholding documents and testimony from Republican investigators pursuing information about the "Fast and Furious" scandal.

Trump himself plans to make that case no matter the parameters of the Senate trial during campaign rallies and speeches in the new year. He spent the evening of his impeachment delivering a disjointed and irate speech in Michigan that foreshadowed his messaging going forward.

"Let's impeach him, for that, for the IRS scandal, for the guns," Trump said of his predecessor. "Remember the guns, he was giving them to anybody that wanted them. He gave guns to the worst people in the world and then they didn't have them registered, right? Not to good. Impeach him. Why didn't the Republicans impeach him?"

Republican and White House officials are operating under the assumption that impeachment will grow less popular as the Senate trial gets closer, meaning Trump is not as likely to offer any concessions to Democrats.

"I don't think impeachment will get more popular over the break," a senior Republican official close to the process said.

On the other article of impeachment -- abuse of power -- officials say the White House plans to argue the Democrats have laid out a weak case based on opinions and not facts. White House officials plan to amplify what they will argue are discrepancies in the Democrats' case -- such as the House Judiciary Committee's decision to accuse the President of bribery in its 658-page report despite citing something different, abuse of power, in the article itself.

Lawyers will also argue the Trump was well within his right as the head of the executive branch to withhold aid and propose a White House meeting with Ukrainian President Zelensky.

They have also discussed arguing former Vice President Joe Biden is not immune from scrutiny simply because he may face Trump in the election next year.