At the federal level: Much of the funding for campaigns comes from individuals who donate money. At the federal level, these contributrions are subject to limits, enacted in 1974 in the aftermath of Watergate.
An individual may give a candidate no more than $1,000 per election. The law also caps individual donations to a political party at $20,000.
However, corporate contributions to candidates for President and Congress have been outlawed since 1907; contributions from labor unions have been outlawed since 1943.