I apologize if my answer is too vague, but in a course I took at university years ago I recall a professor mentioning off-hand the way medieval banks would strategically back certain monarchs. In particular, I remember my professor talking about how sometimes, if a monarch was unable to pay back loans, banks would finance their enemies in an attempt to basically get the money owed to them through shear force. Any information on the subject would be greatly appreciated.
This is an interesting question. I would direct you towards the book Medieval Super-Companies for more information on the subject, http://www.worldcat.org/title/medieval-super-companies-a-study-of-the-peruzzi-company-of-florence/oclc/29386865&referer=brief_results. The book is a study of the Peruzzi Company, the second largest compangnia of medieval Florence (the largest being the Bardi) of the early fourteenth century. The great Florentine families of that century did indeed back the great monarchies of Europe. Unfortunately, collecting the debts was rather difficult, and the Peruzzi family found it very difficult to compel Edward III to pay them back for the loans the provided him to finance the "first conflict" of the 100 years war. Interestingly, the author of that book shows that the collapse of these companies had less to do with royals defaulting on loans and more to do with the contours of Mediterranean bulk commerce.
I would be interested in what other people have to say about this question though.
I can tell you, however, that great mercantile families and banking families had a great say in Papal politics. The Peruzzi, Bardi, and Acciaouli families all helped to run the curial finances. I saw a talk by Alan Stahl wherein he demonstrated that the Condulmer family produced two popes and a number of cardinals in three generations. Furthermore, this family was fairly average in terms of the grand scheme of Venetian economics.
Then, there are the Medici popes as well.