When bills are introduced, they are referred to the committee having jurisdiction over that area of legislation, which often holds public hearings to gather testimony, opinions, and information regarding the legislation. Witnesses, who are often experts, make valuable and well-documented presentations, which include analysis from uniquely informed perspectives.
Hearings are also normally included in Compiled Legislative Histories.
Lexis provides the following:
Congressional Hearings 1824 - (In the main search box, type "committee hearings" Select Congressional Hearings 1824 - from the sources listed in the drop down menu)
CQ Congressional Testimony 1993 - (In the main search box, type "committee reports" Select CQ Congressional Testimony from the sources listed in the drop down menu)
ProQuest Congressional provides:
Hearings, 1834 -
Go to Advanced Search, select Hearings 1834 - Present. Recommended: When searching, use the "All fields including full text" option, from the pull down search menu.
Congress.gov offers hearing transcripts beginning with the 103rd Congress (1993) forward. For those hearings that have transcripts, you will see “Transcript Available” to the right of the hearing's title in your search results.
GovInfo.gov provides hearing transcripts beginning with the 104th Congress (1995) forward. Select transcripts are available from earlier years, and digitization of prior Congresses' hearing transcripts is an ongoing project.