JACKSON, Miss.—Republican presidential nominee Donald Trump proposed contradictory immigration policies, saying in a broadcast interview that it would be “a very, very hard thing” to deport illegal immigrants who have been in the country for decades.

During the primaries, he called for a “deportation force” to remove all 11 million undocumented immigrants, and a campaign aide said Wednesday his position hasn’t changed. He also said this to Fox News host Sean Hannity on Tuesday about people who have overstayed their visas:...