At certain points in the campaign, yes. That's one of today's rulings.
It already happens with the Federa Matching Fund scam. If you accept matching funds, you run into problems if you need to make a push in this or that state late in the primary campaign. Or, once that stage is over, if you need to get your message out during the summer. That's why Bush decided not to accept the funds: because he didn't want the restrictions.
Dean is doing the same thing this year.
One major effect of these spending restrictions is that they protect incumbents and frontrunners, to the disadvantage of challengers.
Why should that be? What good is served?
And yet, in exchange, we get restrictions on our free political speech.