WASHINGTON (AP) — Vice President Joe Biden must be everything President Barack Obama was not: energetic, aggressive and concise. Rep. Paul Ryan must meet the moment and keep things going Mitt Romney's way. And voters must figure out whether an evening with the running mates even matters to them at all.

In an election running short on time for turning points, Thursday night's vice presidential debate has the potential to be one. But history suggests otherwise. Just as vice presidents play a secondary role, so do the debates between the No. 2 candidates, particularly with two face-offs remaining between the men who would lead the nation.

What matters this time is which candidate delivers a more compelling message about helping Americans get jobs or improve their standing in life.