Concededly, neither the limitations on contributions nor those on expenditures directly or indirectly purport to control the content of political speech by candidates or by their supporters or detractors. What the Act regulates is giving and spending money, acts that have First Amendment significance not because they are themselves communicative with respect to the qualifications of the candidate, but because money may be used to defray the expenses of speaking or otherwise communicating about the merits or demerits of federal candidates for election. The act of giving money to political candidates, however, may have illegal or other undesirable consequences: it may be used to secure the express or tacit understanding that the giver will enjoy political favor if the candidate is elected. Both Congress and this Court's cases have recognized this as a mortal danger against which effective preventive and curative steps must be taken.