I believe it really becomes a question of the viability of the campaign itself, as opposed to.... It may sound odd to say there doesn't have to be a connection between the viability of the campaign and the electoral outcome, but it is a question.... There is a range of factors in assessing a loan, and that would be the case in political financing as well.
Is the campaign viable? It comes down to asking if it can generate revenue through fundraising in order to repay its debt.
The rebate is absolutely an element, as you've pointed out. If that's going to come into the discussion for the terms of the loan, then assessing whether a rebate is a potential would have to be part of the equation
It's a combination of factors. A campaign's financial viability on its own merits is something a bank would be interested in.