No, says the Tenth Circuit in Riddle v. Hickenlooper (Jan. 23, 2014). Judge Gorsuch’s concurrence summarizes the issue well:

In at least one important way Colorado discriminates against minor party contributors. An example illustrates the problem: contributors to state legislative races can make two separate $200 contributions ($400 in all) to major party candidates who face no primary opposition. One of these $200 contributions may be nominally attributed to the (uncontested) primary, the other to the general election, but under Colorado’s statutory scheme all the money can be spent in aid of the general election contest. Meanwhile, contributors to minor party candidates who face no opposition for their party’s nomination can make only one $200 contribution. So it is that, in the particular situation we face, major party contributors can channel twice as much money to their favorite candidate in the general election as minor party contributors can.

This sort of different treatment, the court concludes, is unconstitutional, given that contributing money to political causes is protected by the First Amendment. Such contributions to candidates (as opposed to expenditures independent of the candidates, or contributions to advocacy groups that are independent of candidates) may be subjected to nondiscriminatory caps — but not to a cap such as this one.

Note that a state can choose to not accept write-ins on its ballots (see Burdick v. Takushi (1992)). But this reflects the state’s considerable power over how it will print ballots and how it will count them. Such managerial authority doesn’t extend to a power to restrict speech about candidates, or restrict the contributions of money that make such speech possible. Such restrictions are subject to a higher level of judicial scrutiny than are ballot access restrictions, and the Tenth Circuit held that this restriction fails such scrutiny.

UPDATE: The original post title was imprecise; I’ve corrected it. Thanks to commenter Srynerson for pointing this out.