First, Senate Democrats will try to recruit Mr Portman, a well-liked moderate in bipartisan Senate circles, to join them in convicting Donald Trump at his impeachment trial for inciting an insurrection at the Capitol on 6 January.
The Democrats need at least 17 Republicans to vote with them to reach a two-thirds majority and convict Mr Trump, barring him from ever holding federal office again.
Mr Portman’s retirement also throws the 2022 Ohio Senate race into flux as Republican voters confront a crossroads: will they turn for good down the populist, anti-institutional trail blazed by Donald Trump over the last half decade, or remain on the old-school conservative route paved by Mr Portman and other establishment figures more aligned with the traditional GOP?
And finally, some prominent Democrats in the Buckeye State will be keen to seize the moment in 2022 now that the party’s candidate won’t be facing a well-established Republican with all the advantages of incumbency.
Mr Portman — who served in the House for 12 years and worked under President George W Bush as a trade representative before coming to the Senate in 2010 — lamented the fierce polarisation that has gripped Washington over the last several years.
“I don’t think any Senate office has been more successful in getting things done, but honestly, it has gotten harder and harder to break through the partisan gridlock and make progress on substantive policy, and that has contributed to my decision,” Mr Portman said in a statement on Monday.
“This is a tough time to be in public service. For many of the issues I am most passionate about, I will continue to make a difference outside of the Senate, beyond 2022,” he said.
“In the meantime, I am hopeful that President [Joe] Biden will follow through on his inaugural pledge to reach across the aisle, and I am prepared to work with him and his administration if he does,” Mr Portman said.
While Mr Portman has carved out a reputation for himself as a serious legislator willing to reach across the aisle for results — he was a key architect of the $908bn Covid relief bill from December — he was never one of the most outspoken Republicans during the Trump administration.
The Ohio senator, like all but one of his GOP colleagues, voted to acquit Mr Trump at last year’s impeachment trial, saying, “While I don’t condone this behavior, these actions do not rise to the level of removing President Trump from office and taking him off the ballot in a presidential election season that’s already well underway.”
The former president had been impeached for withholding key military aid from Ukraine as leverage to strong-arm the new Ukrainian president into announcing an investigation into Mr Trump’s political rival, Mr Biden.
Mr Portman also opposed that first impeachment on procedural grounds, arguing it was too hastily conducted in the Democratic-controlled House and that the article did not accuse the president of any actual crime — a convenient rhetorical off-ramp taken by politicians since time immemorial.
In his speech on the Senate floor in February 2020 explaining his decision to acquit Mr Trump that year, Mr Portman posited the same argument for appeasement that many Republicans have already recycled this time around: Impeachment is too divisive, and voting to convict Mr Trump will only serve to push Americans farther apart.
“Our country is divided and I think the impeachment has further divided an already polarized country. I think we heal in part by surprising the people and coming out from our partisan corners and getting stuff done, stuff that they care about that affects the families we were sent here to represent,” Mr Portman said at the time.
Democrats are hoping Mr Portman (along with 16 other Republicans) changes his tune this time around with a more tangible accusation being levied against Mr Trump.
Senators will be voting to ban a former president from office who has been accused of inciting a bloody insurrection at the very building where they were conducting the crucial business of American democracy: the certification of Mr Biden’s electoral victory.
Shortly after Mr Trump was impeached a second time earlier this month, Mr Portman released a statement saying he would “do my duty as a juror and listen to the cases presented by both sides.”
But Mr Portman added that outside political forces — the impact of a conviction vote on the American social fabric — would influence his decision as well.
“President-Elect Biden has rightly said he wants to set a new tone of greater unity as his administration begins. All of us should be concerned about the polarization in our country and work toward bringing people together. If the Senate conducts an impeachment trial, among my considerations will be what is best to help heal our country rather than deepen our divisions,” Mr Portman said in the 13 January 2021 statement.
The 2022 race
Mr Portman’s exit sets the scene for a lively Republican primary with the GOP undergoing an identity crisis in the wake of the Trump presidency.
Congressman Jim Jordan, the top Republican on the House Judiciary Committee, has built a national brand for himself as one of Mr Trump’s most vicious attack dogs. He is wildly popular in the conservative and right-wing social media eco-system for his loyalty to the ex-president and makes dozens of appearances every year on Fox News.
Mr Jordan led the Republican conference through debate on the House floor during Mr Trump’s second impeachment.
Congressman Anthony Gonzalez, the former Ohio State and NFL wide receiver who was one of 10 Republicans to vote for Mr Trump’s impeachment earlier this month, could also have his eyes on Mr Portman’s seat.
Some in-state officials may also enter the fray, including Lieutenant Governor Jon Husted, Secretary of State Frank LaRose, and Attorney General Dave Yost.
The preponderance of statewide GOP officials — Governor Mike DeWine is also a Republican — shows how Ohio is no longer the purple swing state it once was.
But it does have a split delegation to the Senate with Mr Portman and the staunchly pro-union Democratic Senator Sherrod Brown.
Among the names being floated for the Democratic nomination are Congressman Tim Ryan, a Rust Belt-style Democrat who briefly ran for president in 2019, and Mayor of Dayton Nan Whaley.