1. Public Opinion Polls: Regularly conducted, well-regarded public polls could provide evidence of Biden’s competitiveness if they show him in a favorable light compared to Trump.
2. Policy Successes: If Biden is able to pass important legislation, solve significant problems or accomplish his stated goals, these could help convince Democratic leaders of his ability to compete effectively.
3. Debates: Many people gauge the effectiveness and electability of a politician by their performance in debates or public speaking engagements.
4. Fundraising: The ability to raise substantial campaign funds is a concrete indicator of a candidate’s viability and competitive ability.
5. Endorsements: Having the support of well-respected figures within and outside the party could help prove Biden’s ability to beat Trump in a potential head-to-head match-up.
However, keep in mind that political outcomes are deeply complex and influenced by a multitude of factors so these indicators should not be considered definitive proof.