Dr Mehmet Oz, a Turkish-American TV personality who made more than 60 appearances on Oprah Winfrey's show before it stopped airing in 2011, is running to replace retiring Republican Pat Toomey in Pennsylvania.
While Mr Trump has not confirmed his intention to run for president again in 2024, he is widely expected to, and therefore his endorsements are being closely watched.
Dr Oz was said to have enjoyed the media attention he received and went on to appear as a health expert on Winfrey's show for five seasons, before she offered to produce a series of his own. His public image took a blow in 2014, however, when he told US politicians investigating bogus diet product ads that some products promoted on his show lacked"scientific muster".
Sean Parnell had been considered the favourite after receiving Mr Trump's first endorsement last September, but he suspended his campaign in November after he lost custody of his three children following allegations that he had physically abused his estranged wife.