Running mates don’t usually matter. Kamala Harris might.

Vice presidential contenders do not have a rich history of altering the course of an election. But Republican and Democratic political professionals alike believe Harris’ selection may carry more weight this year. That is because Biden, despite decades of government work, remains less sharply defined to voters than previous presidential nominees. And the rush to test the theory that Harris might matter is opening a critical new stage in the campaign.

“Look back four years ago, people had a pretty clear view of Hillary Clinton — it wasn’t always positive,” said Jeff Link, a veteran Democratic strategist who has studied voters who flipped from Barack Obama to Trump in 2016. “So, Tim Kaine’s attributes never rose to the level of their information about Hillary.”

But Biden is another story. Whole swaths of voters know little more about him than that he was Obama’s vice president, according to Link and other Democrats who have conducted focus groups in recent months. And Biden, who will turn 78 on Nov. 20, has referred to himself as a “transition candidate,” putting more weight on Harris, who ran for president last year and is widely expected to run again.