In Congressional Debate, the use of data, or evidence, is expected.
Evidence can take many forms: statistics, expert testimony, and specific examples are some of the most common.
Because you are not an established expert, you cannot simply argue for a position; no matter how reasonable your arguments may be. Thus, it is your obligation to conduct extensive research to prepare for the topics you will debate.
In an ideal situation, evidence would be provided whenever the speaker makes a claim about the world. Since we are dealing with limited time for a debate speech, we must make choices about when to provide evidence and when to cite sources. If information is likely to be challenged, the speaker should provide a citation; for example, probably dozens of estimates of future federal deficits exist, and so the source of the estimate becomes more important.
Source citations are not necessary when ideas or data are so widely available that they are common knowledge. One rule of thumb is that if a piece of data can be found in 10 different sources, a citation is not necessary because no one is likely to challenge the information and because the information can be said to be part of the public domain.