They should be able to articulate programmes and reasons for supporting their side.
For Kaine, the Virginia Senator, his job will be to maintain the momentum for the Democrats and push the argument that Trump is unfit to be president.
Pence will try to put a calm, respectable, likeable face on the Republican campaign and some meat on the bones of Trump's policies. He will try to reassure anti-Trump Republicans and independents.
Kaine will be trying to reach out to groups - Millennials, men without university education, Hispanics, independents, Republicans - the party is trying to mine for turnout.
Join us for the debate from 2pm. It is coming from Longwood University in Farmville, Virginia, and is being moderated by Elaine Quijano of CBS News.
It will be divided into nine segments of about 10 minutes each. After the moderator asks a question, each candidate will have two minutes to respond, with the moderator using the remaining time for a deeper discussion.