Joni Ernst does a walk through Friday of the venue where Donald Trump will speak Saturday. (Photo by James Hohmann/The Washington Post)

DES MOINES — The night before Donald Trump flies to Iowa to speak at a motorcycle rally she’s organizing, Republican Sen. Joni Ernst called on the presidential candidates to “tone it down.”

Ernst, who was briefly under consideration to be his running mate, declined to directly weigh in on the Republican nominee calling Hillary Clinton “a bigot.”

“I’m not going to get into that,” she said Friday evening. “To both of them, I’d say, they need to take this into a civil discourse. I don’t like it when campaigns go that direction. I’d say to both of them, back down. And let’s really talk about the policies and the issues. That’s my advice to them.”

Ernst wore black denim jeans and a Harley-Davidson T-shirt as she walked through a pavilion on the state fairgrounds where Trump will speak. Volunteers were pitching hay bales to build a stage as she spoke with a handful of reporters.

Asked about Trump’s divisive rhetoric, Ernst said he would be much better served by “really focusing” on the economy, the Iran nuclear agreement and parochial Iowa concerns related to agriculture – in lieu of name calling.

“He needs to say a lot of things, but I think focusing on Iowa right now is going to be very, very important,” she said. “He needs to talk about what we can do in the state.”

Ernst also said she looks forward to Trump taking a firm position on immigration. “I’ve heard he’s announcing his plan next week,” said Ernst. “I don’t know, but I am excited about looking at what he’s proposing next week. I think that will be an important topic coming up here in the election, to put up Trump’s plan against Hillary Rodham Clinton’s plan. But I don’t know what it is.”

Trump lost the Iowa Republican caucuses in February to Ted Cruz, but he has stayed competitive in the context of a general election. The last three statewide polls show the race within the margin of error. A Suffolk University poll two weeks ago found Trump up 1 point.

The senator will lead a procession of motorcyclists on a 42-mile trek through the countryside Saturday morning. They will finish at the fairgrounds, where Trump will speak around 1 p.m. Central time.

Ernst smiled when asked if Trump will ride a motorcycle, perhaps envisioning the scene. “I will be the only one riding,” she said.

The afternoon rally with the GOP nominee was supposed to be outdoors, but rain and an uncertain forecast for more prompted organizers to move the program indoors.

“It was a little muddy and messy,” Ernst said.

Other politicians scheduled to speak include senior Sen. Charles E. Grassley, who has a tougher-than-expected reelection race, and Sen. Deb Fischer (R-Neb.). Iowa Gov. Terry Branstad, whose son Eric is leading Trump’s effort in the state, also plans to appear.

Ernst said she’s happy to campaign with Trump when it works with her schedule. “I think he’s doing quite well in Iowa, and I’m hopeful,” she said. “He speaks in a way that resonates with millions of Americans. It’s exciting to see him shaking things up a little bit.”

The 46-year-old argued that Trump has a more enthusiastic base of supporters than Clinton does in the state.

Ernst declined to say how many attendees are expected. Proceeds from Saturday’s event will benefit a charity called Soldier Strong, which helps paralyzed soldiers walk again.

This is Ernst’s second “Roast and Ride,” and she hopes to establish it as a major annual tradition. It’s inspired by the steak fries that former Democratic senator Tom Harkin put on every summer for 37 years before retiring. Ernst picked up his seat in 2014.

Harkin always served beef, but she’ll serve her supporters roast pork on a bun.

This is fitting since Ernst first came to national attention with a commercial that featured pigs.

“I grew up castrating hogs on an Iowa farm, so when I get to Washington I’ll know how to cut pork,” she said to the camera.