Sen. Rob Portman (R-Ohio) speaks with college students who volunteered to help call voters at his Columbus headquarters on Dec. 6. (Ty Wright/The Washington Post)

Sen. Rob Portman is angry with what he derides as “dysfunctional Washington.” He wants to get tougher with China over trade and currency manipulation. He’s working on a tax reform plan with none other than Carl Icahn, the business magnate whom Donald Trump labels “a killer” and floats as his treasury secretary.

But Portman also stresses that he disagrees with the more extreme parts of Trump’s agenda, such as banning Muslims from entering the United States. “I’m not a rabble-rousing, red-meat Donald Trump guy,” he said, not that anyone would mistake the mild-mannered Ohio Republican for a Trump clone.

To see how the prospect of Trump becoming the Republican presidential nominee is altering the nation’s political calculus, look no further than Ohio. Or New Hampshire. Or Pennsylvania or Wisconsin or Illinois.

All are states where polls predict that Trump would struggle in a general election and where analysts say the shrapnel from Trump atop the ticket would injure Republican lawmakers in difficult reelection fights, thus giving Democrats the Senate majority.

That’s why Portman has recalibrated his strategy to prepare for what many party leaders see as the worst possibility: a Trump nomination, or for that matter a Ted Cruz or Ben Carson nomination.

Republican presidential candidate Donald Trump speaks before the Greater Charleston Business Alliance in South Carolina this fall. (Charles Ommanney/The Washington Post)

“Republicans will be talking about all sorts of crazy things; Democrats will be talking about crazy things,” Portman said in an interview this month at his Columbus headquarters. “We’re going to be focused on, ‘Okay, how do you take all this rhetoric and red meat out there and actually get something done?’ ”

Portman has a head start on grass-roots organizing, building coalitions of blacks, women and college students probably gravitating toward the Democratic presidential nominee, especially in a contest with Trump, but who could be persuaded to vote for Portman, too.

“Since Day One, we’ve focused on how do we win if Hillary wins,” Corry Bliss, Portman’s campaign manager, said in reference to Democratic front-runner Hillary Clinton.

For Portman — as well as for senators Kelly Ayotte (R-N.H.), Patrick J. Toomey (R-Pa.), Ron Johnson (R-Wis.) and Mark Kirk (R-Ill.) — 2016 presents a far more daunting task than the one they faced in 2010, when they united conservatives under a “Stop Obama” banner and rode the tea party wave into office.

Now, in the twilight of the Obama era, the five must navigate sweeping unrest in the country about the terrorist threat, stagnant wages and cascading cultural changes; a roiling Republican base fed up with elected leaders; the prospect of high Democratic turnout to elect the first woman president; and, even if a more mainstream candidate is the GOP nominee, the likelihood that Trump still will be shouting from the sidelines.

The obstacles are compounded in Ohio, the quintessential swing state that will be inundated next summer and fall with presidential candidate advertisements and visits — and therefore more difficult for a down-ballot candidate such as Portman to break through.

“It’s really hard to see Republicans keeping the Senate if Trump is the nominee,” said Jennifer Duffy, a Senate analyst at the nonpartisan Cook Political Report. “Portman and Ayotte could run perfect campaigns, their challengers could run lousy campaigns, and they’d still lose.”

Sen. Rob Portman speaks to members of the Beth Jacob Modern Orthodox Congregation in Columbus, Ohio, on the first day of Hanukkah on Dec. 6. (Ty Wright/The Washington Post)

Portman’s mission is to cement his own identity separate from his party’s presidential standard-bearer, whoever he or she might be, as a conservative reformer who has broken through Washington’s gridlock to pass legislation and who, on social issues such as gay marriage, is compassionate and open-minded.

“Voters have to understand they don’t need to go to the polls to stop Rob Portman from carrying out divisive agenda items of the presidential nominee,” said Matt Borges, chairman of the Ohio Republican Party.

When the Republican National Convention comes to Cleveland in July, Portman said, he plans to stage his own events, such as building a Habitat for Humanity house, with his own messages — “our own kind of little convention.”

“We’re not going to try to assume it’s going to be a great convention and everybody’s going to be, ‘kumbaya,’ ” Portman said. “We’re going to assume there may be some discordant messages out of there, and we’ll do our own thing and not get dragged into wherever that convention takes us.”

Democrats are laboring to make sure he does. Last week, the Democratic Senatorial Campaign Committee circulated Portman’s past encouraging statements about Trump to Ohio reporters.

“Since the moment [Trump] entered the race, he has been spewing poisonous ideas and toxic proposals that will be roundly rejected by Americans if he becomes the nominee,” DSCC spokeswoman Sadie Weiner said. “Democrats are not going to let Republicans forget what they have said about him.”

She added, “Even if he doesn’t win the nomination, he’ll still be the face of the GOP.”

Republicans acknowledge this threat. In a strategy memorandum that envisions Trump as the nominee, Ward Baker, executive director of the National Republican Senatorial Committee, wrote: “It is certain that all GOP candidates will be tied in some way to our nominee, but we need not be tied to him so closely that we have to engage in permanent cleanup or distancing maneuvers.”

Before dealing with Portman, however, Ohio Democrats have to settle on their Senate nominee. The heavy favorite is former governor Ted Strickland, who is mounting a comeback after losing his reelection in 2010. Strickland has a spirited challenger in P.G. Sittenfeld, a young Cincinnati City Council member who is running to Strickland’s left and using their differences on gun control to gain traction. The primary election is March 15.

Portman has avoided drawing a serious primary challenge, at least so far, despite some vulnerabilities on the right, including his 2013 decision to support same-sex marriage after his son, Will, came out as gay.

Portman has spent years cultivating personal relationships with conservative leaders and activists here to stave off any grumblings. He boasts endorsements across the spectrum, including from Rep. Jim Jordan (R-Ohio), who chairs the House Freedom Caucus, the rebellious conservative wing. “Every living Republican public official in Ohio has endorsed Rob,” Bliss said, only half-joking.

Bliss estimated that the Ohio Senate race could cost upwards of $100 million. Portman, one of the GOP’s top fundraisers, had more than $11 million on hand as of Sept. 30. Mitt Romney, a friend and the 2012 GOP presidential nominee, was in Cincinnati Dec. 5 raising money for Portman.

A pro-Portman super PAC — helmed by Romney adviser Beth Myers, veteran Ohio operative Tom Whatman and finance chair Chris Collins — is primed to run ads next year drawing contrasts between Portman and his Democratic opponent.

The Portman campaign is attempting to identify Ohio voters who will probably vote for a Democrat for president but might cross over in the Senate contest. “We’re not talking to solid Republicans,” Portman said. “We’re not talking to solid Democrats. We’re talking to people in the great middle.”

For instance, Portman has built a list of 45,000 Ohio Democrats who strongly oppose the nuclear agreement with Iran. These voters should expect to hear a lot over the next year about why Portman opposed it, too.

The senator is trying to make inroads with Jewish Democrats. At sundown on a recent Sunday, the first night of Hanukkah, Portman, who is a Methodist, came to a Columbus synagogue, donned a yarmulke and spoke about the U.S.-Israel alliance at a menorah-lighting ceremony.

“I don’t believe we got a good agreement,” he said about the Iran deal. “I don’t think America led. . . . I think for that reason Israel is less safe, and we are less safe.”

He won some adherents.

“I’m one of those people who doesn’t have a home in either party, but I support Rob Portman,” lawyer Daniel Kayne said. “He’s a thinker, and he does the right thing. He’s a politician with a heart.”

Portman has opened five offices across the state, and his volunteers have made nearly 1 million calls or in-person visits to voters since May — an unusual feat for a Senate campaign one year before the election. The objective is to amass data about these people to help target messages to them next year.

In the interview, Portman was asked how he — a former congressman and former George W. Bush Cabinet official, the very embodiment of official Washington — can navigate the age of the political outsider.

“I share that frustration, I really do,” he said. “I shared it long before it became Donald Trump’s message, which is, ‘Washington is broken.’ I think it’s dysfunctional. And I not only complain about it, but I do something about it. I try to break through.”

Costa reported from Washington.