WEST DES MOINES, Iowa — Representative Alexandria Ocasio-Cortez and the filmmaker Michael Moore held a rally in Iowa City for Bernie Sanders. Amy Klobuchar’s daughter, Abigail Bessler, hosted a “Hotdish House Party” in West Des Moines. And Julián Castro, a former presidential candidate himself, hit the campaign trail for Elizabeth Warren.

Political surrogates have always played a role in presidential elections, by turns lending campaigns credibility, enthusiasm and star power. Beyoncé, Jay-Z and Chance the Rapper held a concert in Cleveland for Hillary Clinton in 2016. The actor Jon Voight and the singer Kid Rock campaigned with Mitt Romney in 2012. Oprah Winfrey stumped for Barack Obama in 2007.

Yet with President Trump’s impeachment trial suddenly forcing the senators in the Democratic race off the campaign trail for the second week in a row, their stand-ins have become something else: the main attraction.

Three leading candidates who are hoping for strong performances in Iowa and other early states are stuck in Washington — Mr. Sanders, Ms. Warren and Ms. Klobuchar. So voters are instead attending meet-and-greets, house parties and canvass kickoffs with actors, politicians and celebrities, as well as candidates’ family members.