Visa is often touted as a great investment because it does not make/hold loans to its card users. This is deceptive, because Visa is reliant on the issuing banks' continuing willingness to underwrite large lines of consumer credit. If issuing banks (i.e. Citigroup, Bank of America, J.P. Morgan) decide to scale down their consumer credit lending, Visa WILL be impacted by lower card user purchasing power and fewer new card accounts.
From http://collegeanalysts.com/2008/05/19/reason-to-be-cautious-on-visa-v-mastercard-ma/