The overwhelming majority of Republicans -- 44 senators -- voted against proceeding with Trump's impeachment trial, but one Republican unexpectedly sided with Democrats.

Sen. Bill Cassidy of Louisiana joined Sens. Mitt Romney of Utah, Lisa Murkowski or Alaska, Susan Collins of Maine, Pat Toomey of Pennsylvania, Ben Sasse of Nebraska who voted against dismissing the trial altogether last month.

"The fact that you have six Republicans now willing to go forward is telling, but of course, six is still a far cry from the 17 that they will need," said ABC News Senior White House Correspondent Mary Bruce, referring to the number needed to impeach Trump. "Even those Republicans who have condemned strongly the president's actions, who have said he bears responsibility that he provoked the mob that marched on the Capitol, still have argued that they do not feel that this is constitutional and that gives Republicans, some political cover in many ways.

"It will be interesting to hear from Sen. Cassidy certainly why he changed his mind or if that gives Democrats any hope that they may be able to change some other minds in the coming days," she said.

Asked earlier in the day if the House managers' presentation impacted him, Cassidy told reporters, "I’m trying to digest facts, and I thought the arguments they gave were strong arguments. Clearly, we have to hear the opposition, but they presented very good arguments" on the constitutional question, he said.

The Senate voted 56-44 to proceed with the impeachment trial.

Six Republican senators voted with Democrats that the trial is constitutional. Last month, 45 Republican senators voted to dismiss the trial in an effort led by Republican Sen. Rand Paul of Kentucky.

Before the Senate began voting, Rep. Jamie Raskin, D-Md., waived the House impeachment managers' remaining time and said, "nothing could be more bipartisan than the desire to recess."

The trial is adjourned until noon Wednesday.

In an intense, gloom-filled presentation, Trump attorney David Schoen called the trial of the former president an “irretrievably flawed process” that “we all will regret forever” and leave a lasting “stain” on the Senate.

Multiple times, he called the impeachment proceedings "a dangerous snap impeachment."

Schoen warned that Democrats were intent on going after Trump to deprive him from office “in the name of the Constitution.”

“But this is an affront to the Constitution, no matter who they target today. It means nothing less than the denial of the right to vote and the independent right for a candidate to run for elected political office guaranteed by the First and 14th Amendments to the United States Constitution, under the guise of impeachment as a tool to disenfranchise," he said, at one point holding up a copy of the document.

In arguing against the House managers’ claim that the trial is constitutional, Schoen continued a line of argument that Trump attorney Bruce Castor started -- that if Trump had done anything wrong, like incitement, then a criminal court proceeding is the correct remedy.

“The risk to the institution of the presidency and to any and all past officers is limited only by one's imagination. The weakness of the managers' case is further demonstrated by the reliance of unproven assertion that if President Trump is not impeached, future officers who are impeached will evade removal by resigning, either before impeachment or Senate trial," he said. "We have a judicial process in this country. We have an investigative process in this country to which former office holder is immune. That's the process that should be running his course.”

Schoen suggested that House Speaker Nancy Pelosi intentionally delayed the article -- and in doing so, did harm to Trump, since the Chief Justice, an impartial arbiter, cannot preside, making the trial unconstitutional, he said.

Instead, Schoen said, a partisan Sen. Patrick Leahy, D-Vt., is judging Trump -- a partisan who Schoen noted has taken a position on impeachment already.

-ABC News' Trish Turner

Both Trump's legal defense and the House managers have wrapped up their arguments over the constitutionality of the impeachment.

The Senators are voting on whether or not the trial is constitutional.