If you have the name of a particular federal law, you can look up the co-sponsors of the law to see if it tends to be favored by conservatives or liberals in Congress.gov.
ProQuest Congressional provides hearings and other documents that shed light on who supports or opposes pieces of legislation.
Additionally, you can look up the topic in policy analysis databases. These will provide analysis and reports from various non-profit and governmental agencies. By looking up the political leanings of those groups, you can get a good sense of who favors the policy.