Almost two-thirds of Americans believe Joe Biden will not finish a four-year term in the White House if he is elected in November, making whomever he picks as his vice president the ensuing president, according to a Rasmussen Reports survey.

Biden is expected to announce his running mate this week ahead of the Democratic Party National Convention, and he has vowed to pick a female running mate.

Biden would be 78 when sworn in, and concerns about his age and health have left many wondering if he would finish a full term if elected. Thirty-eight percent of those surveyed, including 20% of Democrats said they believe Biden is suffering some form of dementia.

According to the Rasmussen survey, 59% of likely voters believed his vice president would be called up to the top office before Biden served out four full years. Thirty-nine percent said it was very likely, though 35% considered it unlikely.