Representative Tim Ryan of Ohio plans to run for his state’s open Senate seat, Democrats who have spoken with him said, a bid that would test whether even a Democrat with roots in the blue-collar Youngstown region and close ties to organized labor can win in the increasingly Republican state.

Mr. Ryan, an 18-year House veteran, has reached out to a host of Ohio and national Democrats in recent days about the seat now held by Senator Rob Portman, a Republican who stunned officials in both parties by announcing last week that he would retire.

Former Gov. Ted Strickland of Ohio, a Democrat who has been encouraging Mr. Ryan to run, said of the congressman, “I think he is the person with the best chance, given this political climate we’re in and given the way Ohio has been performing.”

“He has the ability to appeal to a lot of independents, and Democrats will be very excited about this candidacy,” Mr. Strickland said.

Mr. Ryan has also discussed his candidacy with Representative Marcy Kaptur, the longest-serving member in Ohio’s congressional delegation, and national labor leaders, including Lee Saunders of Afscme, while also receiving a nudge from Hillary Clinton.

Asked about these conversations, Mr. Ryan said on Monday that he was “encouraged by their support, enthusiasm and commitment,” adding, “The U.S. Senate needs another working-class voice, and I’m very serious about the opportunity to continue representing the people of Ohio.”

He is expected to declare his candidacy by the beginning of March, according to Democrats briefed on his planning.

Long one of the country’s quintessential political battlegrounds, Ohio has turned sharply right since former President Donald J. Trump’s ascent. Mr. Trump carried the state by eight percentage points in 2016 and won it again by the same margin last year, even as Joseph R. Biden Jr. emphasized his working-class appeal and made a late push in the state.

Senator Sherrod Brown is the only Democrat remaining in statewide office in Ohio. And even with his fiercely populist approach, Mr. Brown has lost ground among once-reliable Democrats in eastern Ohio, including those in the industrial area south of Lake Erie and in the more rural enclaves that trace the Ohio River.

Mr. Ryan hails from Niles, Ohio, just north of Youngstown, a region filled with voters who are effectively Trump Democrats, many of them union members or retirees. He outperformed Mr. Biden in his district, but Democrats there suffered a series of losses in other down-ballot races.

The question, should Mr. Ryan become his party’s nominee, is if he can win back these mostly white voters.

Mr. Ryan has long considered running statewide, but in the past decided on seeking re-election to the House seat he first won in 2002, when he succeeded the famously fiery, and corrupt, James Traficant.

Mr. Ryan mounted a long-shot bid for the presidency in 2019 with the same message he’s expected to carry into the Senate contest — that Democrats will build enduring majorities only if they reclaim support from a multiracial, working-class coalition of voters.

Beyond elevating that argument, Mr. Ryan, 47, has another compelling reason to run for the Senate: As Republicans grow stronger in eastern Ohio, his district has become increasingly competitive, and the Republican Party could redraw the state’s districts to make it even more forbidding for him in 2022.

While he has risen on the Appropriations Committee, Mr. Ryan has mostly given up on his hopes to join the House leadership, having been turned back in his 2016 challenge against Nancy Pelosi, then the minority leader.

In Congress, Mr. Ryan has been a close ally of unions and has generally toed the Democratic line, shifting toward a stance in support of abortion rights in recent years. Even before formally announcing his bid, Mr. Ryan drew support from the state chapter of the International Brotherhood of Electrical Workers, which on Monday released a letter endorsing his undeclared candidacy.

Mr. Ryan will enter the Senate race as an early front-runner. He is one of the few Democrats left in the state’s congressional delegation, and represents a region of the state the party is desperate to reclaim. He also has deep relationships with national leaders.

On Saturday, Mrs. Clinton publicly encouraged Mr. Ryan to run for the Senate, repaying him for his support for her when she ran against Barack Obama in the 2008 presidential primary race.

“You’re right, Kathy!” Mrs. Clinton wrote on Twitter, promoting a message from a Democratic activist in Ohio, Kathy DiCristofaro, who wrote that “Ohio needs leaders like @timryan to fight for working people.”

Mr. Ryan also has an ally in the White House, having endorsed Mr. Biden in November 2019, a low ebb in the race for the candidate.

It’s unlikely, though, that the congressman will run unopposed for the Senate nomination. One Democrat whose name has been floated for the seat, Mayor Nan Whaley of Dayton, said she was “thinking about it” when asked on the day Mr. Portman announced his retirement. Ms. Whaley is also considering a run for governor, though, and many Ohio Democrats believe she and Mr. Ryan would try to avoid clashing in a primary.

Equally intriguing to some Democrats in the state is Dr. Amy Acton, who as the former director of Ohio’s Department of Health ran the coronavirus response effort last year for Gov. Mike DeWine, a Republican. She is considering joining the race, The Plain Dealer of Cleveland reported last week, and received her own online boost when Connie Schultz, a longtime Ohio columnist and the wife of Mr. Brown, wrote on Twitter: “Imagine Dr. Amy Acton as Ohio’s next U.S. senator. I sure can.”

The Republicans are likely to have an even more crowded primary field. The race appears to be wide open after the announcement last week by Representative Jim Jordan, the far-right Trump ally whom the former president awarded the Presidential Medal of Freedom, that he would remain in the House.

A number of other House members may run, including Representative Steve Stivers, a Columbus-area lawmaker. A host of would-be self-funders are also eyeing the seat, including Jane Timken, the chair of the Ohio Republican Party.