Senator Portman wrote in a newspaper editorial that Mr Trump’s administration should now release federal funds to ensure a proper transition.
Mr Portman said he supported the president’s right to investigate the election results but that there had been “no evidence as of now of any widespread fraud or irregularities that would change the result in any state.”
Mr Portman was a co-chair of Mr Trump's re-election campaign in Ohio but said the time had arrived for the country to move on.
"Based on all the information currently available, neither the final lawful vote counts nor the recounts have led to a different outcome in any state. In other words, the initial determination showing Joe Biden with enough electoral votes to win has not changed," Portman wrote in an opinion piece in the Cincinnati Enquirer.
"I voted for President Trump ... But I also believe that there is no more sacred constitutional process in our great democracy than the orderly transfer of power after a presidential election. It is now time to expeditiously resolve any outstanding questions and move forward.
"The Biden team should receive the requested intelligence briefings and briefings on the coronavirus vaccine distribution plan. This is only prudent.”
Mr Portman’s editorial comes the day after Senator Lisa Murkowski took to Twitter and urged Mr Trump to begin the transition process and condemned his campaign to influence the outcome of the election.
The Alaska senator said Mr Biden had beaten Mr Trump fairly and told the president it was time to give up his efforts to stay in power.