The day got off to an auspicious start for Geoff Diehl, the Republican underdog running against Elizabeth Warren for Senate.

Around 8 a.m., Diehl and his small team pulled up to the commuter rail stop in Stoughton to greet commuters boarding Boston-bound trains. His volunteer driver, “Capitan’’ Frank Collins, parked the campaign’s signature RV in front of a local business, figuring it was OK since the hours on the door indicated it would be closed awhile longer.

The business owner arrived and pulled out his phone to call his brother, a big Diehl fan, to come. The brother showed up, wrote the candidate a $400 check on the spot, gave him addresses for a dozen relatives who’d welcome Diehl signs in their yards, and offered a word of encouragement: “My whole family is voting for you.’’

He’s trailing Warren by more than 20 points in public polls. He’s trailing her in the money race by millions and millions of dollars. But mile by RV-driven mile, Diehl is taking on Warren by trying to show that he is the one who cares about Massachusetts while she flies across the country campaigning for Democrats in other states or fights with President Trump on Twitter.

He cares so much, he is standing out on a train platform on a Thursday morning that is sunny but only in the mid-30s, shaking hands and doling out campaign literature. Behind him, a lone aide holds up a single “Diehl for Senate’’ sign on a long stake.

He’s easily done more than 50 of these early-morning campaign stops in the 17 months since he got into the race, Diehl said. He finds it a good way to talk with people without interrupting them. They’re a “captive audience,’’ he joked, with time to read his literature on the train.

The 9:15 a.m. train pulled out of the station, and it was time to get back on the RV. The campaign bought it for about $20,000 off Craigslist at the beginning of last summer. A donor helped pay to have it wrapped in bright red, white, and blue “Diehl for Senate’’ graphics, complete with a giant picture of the smiling candidate, arms folded, shirt-sleeves rolled up. “Geoff Saved Taxpayers $2 Billion!’’ declares the mobile billboard-cum-transport, a reference to the successful ballot initiative Diehl led in 2014 that repealed automatic gas tax increases. Lettering over the giant windshield boasts, “The Real Diehl.’’

Inside, a supply of bumper stickers — “Stop Elizabeth Warren,’’ reads one flavor — is stuffed into the kitchenette sink. In the back, bedroom amenities have been removed to make storage for signs, banners used in parades, and other campaign gear.

Traveling this way has its advantages. Cars honk their support — as happened about a dozen times over the course of the morning, as the RV traveled from Stoughton to New Bedford to Plymouth. Once, stuck in heavy Cape traffic, he passed literature and bumper stickers through the window to a car full of women on their way to a concert.

The conversation turned to his opponent, and his argument that Warren is contributing to the “toxic stew’’ and “destroy at all costs’’ mentality that dominates Washington.

But what about President Trump, whom Diehl supports? Diehl noted that the president condemned political violence and the actions taken by the person — at that time, still unidentified — who’d mailed pipe bombs to prominent Democratic figures and CNN.

“I conduct myself in a whole different way than the president,’’ he said, when pressed further. He quickly pivoted back to Warren. When congressional Republicans were working on legislation to repeal the Affordable Care Act, she described them as seeking “blood money’’ to pay for tax cuts, even though a gunman — a supporter of Bernie Sanders, Diehl noted — had just shot Louisiana Representative Steve Scalise, he said.

“If you villainize a political party or any one individual you’re going to set off an element in our society that may act irrationally,’’ he said.

But if it’s wrong for Warren to use such rhetoric, isn’t it also wrong for Trump?

“Yeah, I mean, it’s not the dialogue that’s going to help us come together, I think. Here she is complaining about him, and yet she follows the same style, and ultimately wants to replace him,’’ Diehl said.

In New Bedford, the richest seaport in the nation, Diehl toured the 90-foot-long Norseman, a scallop vessel owned by Eastern Fisheries Inc., which bills itself as the world’s largest harvester, processor, and marketer of the lucrative shellfish. He examined the steel dredges used to sweep up scallops from the ocean floor, saw the steel trough where the crew shucks each scallop by hand while still at sea, and heard about federal regulations that are putting the fleet at a competitive disadvantage.

Diehl promised the company’s executive vice president, Joe Furtado, that “if this works out’’ he’d be back to New Bedford the day after the election to talk more about what he can do to help out the industry, which he argues Warren has ignored.

Back on the road, another car honked as it passes. That was a Prius, an aide joked.

“I’ll tell you what it wasn’t. It wasn’t a Subaru,’’ offered Collins, the driver.

“OK,’’ Diehl said, in a parental tone. “Not that we profile,’’ he joked.

Along the way, Diehl pointed out local landmarks and arcana. He recounted the crime problems at the Silver City Galleria mall in Taunton and the history of Route 18 in the whale oil trade, which, he continued, is part of Massachusetts’s history of privately owned roads and enterprises that helped build the state, not solely the government as Warren claimed in her 2012 campaign. He discussed the pros and cons of wet versus dry cranberry harvesting as the RV barreled by roadside bogs.

The RV pulled into the parking lot of a Plymouth hotel hosting the annual meeting of the Massachusetts Police Association, the state’s largest law enforcement union, with 18,000 members.

Diehl stood along the wall of the hotel ballroom as the police band and color guard marched in, and the group’s chaplain recalled the difficult losses the state’s law enforcement community suffered in the past year, including the fatal shooting of Yarmouth police officer Sean Gannon. “The people that criticize you so much wouldn’t last 15 minutes in a cruiser or 10 minutes behind a badge,’’ the chaplain, Father Paul Clifford, said before offering a prayer.

A few minutes later, Diehl was introduced and Revere Police Chief Jim Guido moved for the MPA to endorse him. “Our profession’s under attack and our present US senator doesn’t respect the people in blue, and we all know Geoff Diehl does,’’ said Guido. Another executive seconded, and a thunderous “aye’’ made it official.

Then Diehl was headed back toward the RV.

Holyoke Lieutenant James Albert stopped Diehl as he headed out of the hotel. “Just beat her,’’ he said.

Victoria McGrane can be reached at victoria.mcgrane@ globe.com.