Last year, Sanders attended the White House Correspondents' Association dinner while Trump held a rally in Washington Township, Mich. Trump also skipped the dinner two years ago in his first year in office but attended the 2011 gala when President Barack Obama and comedian Seth Meyers roasted him.
On Saturday night, Trump brought Sanders on the indoor stage early into his rally.
The crowd erupted into cheers of "Sarah, Sarah, Sarah."
"Last year this night I was at a slightly different event, not quite the best welcome," Sanders said. "So this is an amazing honor. I'm so proud to work for the president.
At last year's WHCA dinner, comedian Michelle Wolf roasted Sanders, seated at the head table, as well as Trump. Some attendees walked out.
After Sanders' brief appearance Saturday, more applause followed.
"She doesn't get it. She doesn't understand me after all these years. She's becoming too popular, Sarah, I'm jealous. You're fired!," said Trump, repeating his signature line from his television show, The Apprentice.
During the "Make America Great Again" rally, Trump spoke for 90 minutes.
"There's no place I'd rather be than right here in America's heartland," Trump told the thousands in the audience, some of whom had waited overnight in the cold to be first in line. "Is there any place that's more fun than a Trump rally?"
In 2016, Trump won Wisconsin by 22,748 votes over Democratic presidential nominee Hillary Clinton. During mid-term elections, Democrats swept statewide races, including ousting incumbent Gov. Scott Walker.
He touched on his usual topics: a flourishing economy, the "witch hunt" investigation by Robert Mueller, "extreme" Democrats, immigration and asylum, and took shots at some Democratic presidential contenders.
Trump said it was "actually my sick idea" to send those crossing the border to sanctuary places with laws and regulations that shield undocumented immigrants from being detained, incarcerated or removed by U.S. Immigration and Customs Enforcement.
"Last month alone, 100,000 illegal immigrants arrived at our borders, placing a massive strain on communities and schools and hospitals and public resources, like nobody has ever seen before," Trump said to the crowd. "Now, we're sending many of them through sanctuary cities, thank you very much."
On April 14, Sanders said on This Week the idea "was brought up at a staff level and it was determined at that time that, logistically, there were a lot of challenges and it probably didn't make sense to move forward and the idea did not go further ... "
Journalists and their guests gathered at the Washington Hilton ballroom.
Instead of a comedian, an historian was invited. Ron Chernow, the author of the Alexander Hamilton biography that inspired the hit musical Hamilton, gave some jokes but mainly discussed democracy and freedom of the press.
"We now have to fight hard for basic truths that we once took for granted," he told those attending the event.
He only uttered Trump's name once but he talked about the president's rhetoric, including describing mainstream media as the "enemy of the people."
"When you chip away at the press, you chip away at our democracy," Chernow said.
Trump had instructed members of his administration not to attend the black-tie event.