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Two-term Republican Sen. Rob Portman of Ohio announced Monday that he won’t seek a third six-year term when he is up for reelection in 2022.

Portman touted his legislative achievements in a statement announcing the news, but the senator also pointed to congressional gridlock as a reason he won’t campaign for another term in the Senate. He serves on the Senate Finance, Homeland Security and Foreign Relations Committees.

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"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," Portman wrote.

In this image from video, Sen. Rob Portman, R-Ohio, speaks as the Senate reconvenes to debate the objection to confirm the Electoral College vote from Arizona, after protesters stormed into the U.S. Capitol on Wednesday, Jan. 6, 2021. (Senate Television via AP)

Portman served a decade and a half in the House before serving as U.S. Trade Representative and director of the Office of Management and Budget in President George W. Bush’s administration. He was first elected to the Senate in 2010 and easily reelected in 2016.

Pointing to his three decades in public service, the senator noted that "this was not an easy decision because representing the people of Ohio has been an honor. But I’ve been doing this a long time, longer than I ever intended."

Portman’s decision to retire rather than run for reelection will likely make the GOP’s hopes of recapturing the chamber in the 2022 elections more challenging.

The Senate is split 50-50 between the two parties, but the Democrats hold a razor-thin majority due to the tie-breaking vote of Vice President Kamala Harris. That means Republicans only have to flip one seat in 2022 to regain the majority they lost in the 2020 elections.

But the GOP is defending 20 of the 34 seats up for grabs in the next midterm elections.

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The difficult map isn’t the only obstacle facing the Republicans. They’re also defending open seats in two crucial battleground states due to retirements. Sens. Pat Toomey of Pennsylvania and Richard Burr of North Carolina are not running for reelection. With Portman’s announcement, add battleground Ohio to the list of states with a GOP-controlled open Senate seat.

There’s also a potential headache in Iowa, where 87-year-old GOP Sen. Chuck Grassley is staying mum so far on whether he’ll run for an eighth six-year term in the Senate. And in the battleground state of Wisconsin, Republican Sen. Ron Johnson hasn’t said if he’ll run for a third term.

With Monday’s announcement from Portman, speculation immediately focused on whether Rep. Jim Jordan, the longtime Republican congressman from northern Ohio and close Trump ally and friend, would run for the Senate. Other potential Republican candidates include J.D. Vance - the author and venture capitalist known for his memoir "Hillbilly Elegy," which was a New York Times best seller – and Josh Mandel, the former two-term Ohio treasurer who was defeated by Democratic Sen. Sherrod Brown in the 2012 election, former Rep. Pat Tiberi, current Reps. Warren Davidson and Steve Stivers, Attorney General Dave Yost, and Secretary of State Frank LaRose.

Sparking speculation was GOP Rep. Mike Turner of Ohio. In a statement praising Portman, Turner said that "as numerous people have reached out to me, I will continue to look to the opportunity where I can best serve our community, state and country."

Among the Democrats who may consider a Senate run are longtime Rep. Tim Ryan, who unsuccessfully ran for the 2020 Democratic presidential nomination, Reps. Nan Whaley, Joyce Beatty, and Marcia Fudge, and Cincinnati Mayor John Cranley.

Ryan tweeted Monday afternoon that he's "overwhelmed by supporters who are reaching out to encourage me to run for Senate. I haven’t made a decision yet but I’m looking seriously at it."

Sen. Rick Scott of Florida, the chair of the National Republican Senatorial Committee (NRSC), which is the Senate Republican re-election arm, highlighted in a statement that the "2022 election in Ohio starts today and Republicans will hold that seat."

The NRSC’s counterpart, the Democratic Senatorial Campaign Committee (DSCC), argued that "Portman’s retirement is yet another blow for Republicans as the face a deeply divided caucus."

"The NRSC must now deal with another open seat liability in a major battleground state, setting the stage for what is sure to be a messy and divisive primary," DSCC spokesperson Steward Boss said in a statement.

Fox News' Chad Pergram contributed to this story