Take a look at subject headings for ideas to extend or narrow your search. Put phrases in quotes, and use truncation (an asterisk * in the Catalog) to search for alternative endings and spellings of words (e.g., iraq terror* will pick up terror, terrorism, terrorist, etc., while labo*r Britain politics will get results for both labor and labour).
Princeton owns more than 7.5 million items, but keep in mind that the library can also get you books from around the world. Try searching the collections of Ivy League libraries through Borrow Direct or libraries throughout the world using WorldCat.org.
These are abstracting and indexing databases, which means they only search information about the articles, not the full text. In practical terms, this means that they return fewer results and keyword matches are more likely to be relevant. They're especially good at covering the full range of political science publications, including niche journals and international publications.