Romney-Christie?

According to this CBS story, Dan Balz's book on the 2012 election reports that Mitt Romney was seriously considering selecting New Jersey Governor Chris Christie as the Republican vice-presidential nominee, but was dissuaded by an SEC regulation governing campaign contributions:

In the end, it was money, not chemistry, that kept Christie off the GOP ticket. A "pay to play" regulation from the Securities and Exchange Commission prevented the country's largest banks from donating to candidates and elected officials from states in which big banks were located. If Christie, the governor of New Jersey, were added to the ticket, Romney's campaign would have been barred from accepting any campaign contributions from Wall Street - a critical source of cash for the GOP candidate, formerly a private equity manager.

I found this striking and was curious whether the reg was constitutional, but I can't actually figure out what rule the CBS story is referring to.  I did find this long discussion of the SEC pay-to-play reg, but I can't actually figure out what the story is referring to.  Readers -- any idea?  Is there a rule that stops banks from contributing to home-state candidates?  You were very resourceful about Guthrie.