Trying to figure out what was influencing Congress is a goal of many researchers. The following databases will allow you to search for articles and books that will provide background and context into Congressional actions and proceedings. For current issues, another good source to check are the major newspapers (Washington Post, NYT, etc). Also consider following groups on issues you are interested in--many of these will put out statements on about/for/against legislation.
UC Library Search is the University of California's unified discovery and borrowing system.
Access it directly or from the Library homepage to find most UC books, articles, media, archival collections, and more.
See the UC Library Search User Guide and ask for research help 24/7 for more information.
If an article is relevant to your topic, you want to look at the research it cited (backward citation). But it can also be very helpful to see who has cited it (forward citation). There are several different ways to do this, and the results will overlap -- no single method is comprehensive.
Google Scholar provides forward citations for some articles. It has a broader range of documents included (not just peer reviewed journals, but reports, pre-prints, etc.) and doesn't eliminate self citation or de-duplicate the results.
Web of Science is a vast, multidisciplinary database, which includes the Social Science Citation Index. This index allows you to do a "Cited Reference" search if you select that tab. This will retrieve other articles (from a prestigious list of peer reviewed journals) which have cited the target article, and it also shows the references for the the original article... both forward and backward citation.