Portman will be the featured speaker Saturday at the Butler County GOP's Lincoln Day dinner. The Republican-dominated county north of Cincinnati is home to House Speaker John Boehner, who repeated his opposition to same-sex marriage after Portman's reversal on the issue.
Portman told reporters last week that he began changing his views in 2011 when his college-age son Will told his parents he was gay.
Ohio Republican Party chairman Bob Bennett has said there was a flurry of calls at first upset about Portman's change, but they soon died down.
Secretary of State Jon Husted also will speak Saturday.