Here are some recommendations for tracing research on the role of cash in the 2012 presidential elections.
The Federal Election Committee has a campaign disclosure portal. This has official disclosures from the candidates, data on contributions and expenditure.
PBS Frontline Big Money 2012
has been conducting a major campaign on money in the 2012 elections see its reports and findings on the website. Note some documentary material may not be accesible outside the USA.
Sunlight Commission Follow the Money website has detailed analysis on amouints and sources of money in the races.
There is also a useful blog which highlights the latest stories concerning transparency.
Open secrets Detailed analysis from the Center for Responsive Politics. This site has clear graphs which track top donors, sources and disclosure for the candidates.
New York Times has a brief compare the candidates in the money race graph
The Washington Post has a graph of spending on political ads by each party.
The Wall Street Journal has a feature called the political money ball which has visual diaplays of sources of finance for each candidate.
Democracy info also has good visual displays of sources of money
Political Money line is a subscription dataabase from CQ press . Its website tracks money releated issues in all campaigns. Its site offers free acess to headlines.