Senator Joni Ernst of Iowa will speak at the Republican National Convention, making her among the prominent conservative politicians the Trump campaign has lined up for the event, according to three people with direct knowledge of the convention planning.

Ms. Ernst will be given one of the most coveted speaking slots, an address scheduled during a prime-time coverage period planned by the major television networks.

At the same time the Ernst discussions were formalized, the Trump campaign was trying to resolve a more delicate and difficult situation of what role Senator Ted Cruz of Texas might play at the convention. A senior aide to Donald J. Trump said Wednesday that the two sides were still a long way from a resolution.

Giving a prime speaking slot to Ms. Ernst — who was being vetted as a possible running mate to Mr. Trump but is no longer under consideration — is fitting in many ways, like in possibly helping the candidate neutralize the criticism that he has been insensitive to women.