Academic articles will form the basis of your literature review. In order to do any interesting original analysis later, you need to know the current conversation in academic circles about your topic. Your literature review should primarily use peer-reviewed articles written by professors or other academic researchers.
Using subject-specific databases (eg. Worldwide Political Science Abstracts) acts as an intitial filter for your search, since only political science articles will be included. Using general databases (eg. JSTOR) is helpful for getting interdisciplinary perspectives.
Experiment with different search terms- academic databases tend to be very exact in matching words, so the terms you choose make more of a difference than they do in a Google search.
There are some operators you can use to narrow or broaden your source- AND (congress AND committees), OR (congressman OR representative), "" ("united states"), * (legislat* to search legislator or legislation). You may also be able to find Subject Terms when you look at the record of an article. These can also narrow your search, but know that you need to use the exact terms supplied by the database.
Limit your results to scholarly or peer-reviewed sources. Some of these databases also include magazine articles or other sources that are not subject to the same level of academic rigor. Also keep an eye out for what section of a journal an article was published in- some sections use editorial review rather than peer review.
When you find an article that seems relevant to your research question, follow citation trails to turn it into more articles. Look through the article's reference list, and then check the "Cited By" articles in Google Scholar for more recent work.