The 1976 Supreme Court decision in Buckley v. Valeo determined that a cap was within the known limits of free speech.
The Supreme Court in Buckley v. Valeo held that contribution limits are subject to a more lenient standard of review than expenditure limits because they impose only a marginal restriction on speech and will be upheld if the government can demonstrate that they are a “closely drawn” means of achieving a “sufficiently important” governmental interest.1 Unlike expenditure limits, which reduce the amount of expression, the Court opined that contribution limits involve “little direct restraint” on the speech of a contributor.2 While acknowledging that a contribution limit restricts an aspect of a contributor’s freedom of association by affecting a contributor’s ability to support a candidate, the Court determined that a contribution limit still permits symbolic expressions of support and does not infringe on a contributor’s freedom to speak about candidates and issues.3
If this sounds like splitting hairs, you’re correct. Remember that 1976 was the next election year after Nixon’s reign of corruption and the public mood was wildly favorable for such limits. The parties that brought the suit were a coalition unimaginable today: The lawsuit was filed in the District Court for the District of Columbia, on January 2, 1975, by U.S. Senator James L. Buckley (a member of the Conservative Party of New York State), former U.S. Senator and 1968 presidential candidate Eugene McCarthy (a Democrat from Minnesota), the New York Civil Liberties Union, the American Conservative Union, the Socialist Workers Party, the Libertarian Party, and others. The final decision was one of the oddest in Supreme Court history. Five of the eight justices (Douglas had retired) dissented in part, although the core was allowed through.
Both the right and left argue for overturning the ruling, for different reasons. Those are probably outside of the limits on this thread, though.