Meanwhile, choosing Harris as the vice president will compensate for Biden's age disadvantage. As a politician, Harris is very young - only 55 years old. Her joining can significantly reduce the average age of the team, and make Biden's team seem more vigorous than Trump's. This will increase Biden's appeal to young voters. At the same time, being young means she has made fewer political mistakes - this leaves Trump fewer chances to attack.

Harris' nomination will help Biden to establish an affectionate image. When announcing her nomination, Biden mentioned his late eldest son Beau Biden. In 2016, Biden gave up running for president largely because of the death of his son. In 2020, taking part in the presidential election again is, in a certain sense, a way to fulfill his son's last wish. All these undoubtedly make Biden into a loving father figure.

But still, Harris joining Biden's team brings complications for the Democratic ticket. Although Harris has minority characteristics, the question of her appeal to "Middle America" and identity politics looms. Some white voters may strengthen their sense of identity as the ethnic majority and distance themselves from the Democratic Party. This means that identity politics will be more prominent in the 2020 election. In the long run, political polarization based on ethnic identity is likely to become more wide-ranging in the US.

(The author is director of Institute of Strategic Studies and International Security, Fudan Institute of Belt and Road & Global Governance, professor at the Center for American Studies, Fudan University. opinion@globaltimes.com.cn)