The measure is not about restricting speech, they say, but about getting big money out of politics and attacking corruption. There are two big holes in the argument. When a citizen supports a candidate or a cause, he can give his time and effort to the cause. In either case, he is giving of his means to support his political views. But, retort the reformers, money is different.
Money is bad.
Contribution and spending limits don't restrict political activity, just the role of money. So, there is no First Amendment violation. No regulation of content, just of revenues and expenditures. It would be unthinkable as applied to the media. It should be unthinkable as applied to citizens and candidates too.