The Senate voted Tuesday to move ahead with the unprecedented impeachment trial of former President Trump after listening to nearly four hours of arguments on whether it is constitutional to try a president who is already out of office. 

The vote was 56-44.

The six Republicans who joined with the Democrats on voting to continue the trial were Sens. Susan Collins of Maine, Bill Cassidy of Louisiana, Lisa Murkowski of Alaska, Mitt Romney of Utah, Ben Sasse of Nebraska and Pat Toomey of Pennsylvania.

Cassidy changed his vote from January when he was among the 45 Republicans who supported a motion by Sen. Rand Paul, R-Ky., that declared the trial unconstitutional. Cassidy said he was persuaded by the House impeachment managers' arguments on Tuesday.

"If anyone disagrees with my vote and would like an explanation, I ask them to listen to the arguments presented by the House Managers and former President Trump’s lawyers," Cassidy said in a statement. "The House managers had much stronger constitutional arguments. The president’s team did not."

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Trump's legal team said the trial is unconstitutional because he's no longer in office and can't face removal, which is the standard judgment of an impeachment conviction. 

"President Trump is no longer in office. The object of the Constitution has been achieved. He has been removed by the voters," Bruce Castor, a Trump attorney, said. 

Another Trump lawyer David Schoen said impeachment is moot.

"Presidents are impeachable because they are removable," Schoen said. "Former presidents are not because they cannot be removed."

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House Democrats made the case that not only is there precedent for proceeding with impeachment for a federal official who is out of office, but it's the right thing to do to hold presidents accountable. 

They pointed to the 1876 corruption case of William Belknap, President Ulysses Grant's war secretary, who was impeached and tried by the Senate after leaving office.

The House prosecutors said the Constitution applies to every day a president is in office, and Trump's conduct as president cannot be ignored even if he's since left office.

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"History does not support a January exception in any way," Rep. Jamie Raskin, D-Md., said. "So why would we invent one for the future?" 

The Senate trial adjourned until noon Wednesday. 

Fox News' Adam Shaw and Kelly Phares contributed to this report.