WASHINGTON » 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, tonight’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.

That makes this debate significant.

The impression it leaves will help drive the political narrative for the next five days until Obama and Romney go at it again. Any signature moment or performance — electric or awful — could alter the spirits of the campaigns and the TV ads they run. And, of course, influence the millions of people watching at home.

The rookie on this kind of stage is Ryan, the Wisconsin lawmaker. But the burden is bigger on Biden because of what happened last week.

Obama failed at his central mission of drawing distinctions with Romney in a crisp way that connected with people. The merits of competing tax plans or health care visions do not matter much if a debater meanders into the policy weeds or, even worse, fails to deliver a passionate fight. The president ended up doing both.

Expect Biden to try to rebound in three ways: calling out Ryan on perceived inconsistencies, pounding home Obama’s economic agenda for the next four years and appealing to working-class and older voters with a regular-guy style. His gaffes get him and Obama in trouble, but he is more disciplined in a debate setting.

“Joe just needs to be Joe,” Obama insisted beforehand.

Ryan wants to keep the conversation on the larger economic issues that have defined his career, and on Romney’s main ideas about how to create jobs. Such a big-idea focus could also help him deal with the visual of a 42-year-old newcomer sitting next to a sitting vice president who, at 69, carries off the role as older statesman.

The Obama-Romney debate in Denver a week ago will influence what happens on stage at Centre College in Danville, Ky. Biden will come in prepared to slam Romney and Ryan on charges of hiding their own positions. And Ryan will come in knowing Biden faces higher pressure to perform.