I think it's important to clarify the stance of the National Salvation Front because it is often ambiguously reported. First what they said:
(Reuters) - Egypt's main opposition coalition rejected on Sunday Islamist President Mohamed Mursi's plan for a constitutional referendum this week, saying it risked dragging the country into "violent confrontation".
Mursi's decision on Saturday to retract a decree awarding himself wide powers failed to placate opponents who accused him of plunging Egypt deeper into crisis by refusing to postpone the vote on a constitution shaped by Islamists.
"We are against this process from start to finish," Hussein Abdel Ghani, spokesman of the National Salvation Front, told a news conference, calling for more street protests on Tuesday.
But there has been some doubt whether this is a call for boycott or not. In fact, a vote was held by the NSF in which they had three options: campaign for "no" in the referendum, boycott, or continue to push for the referendum to be postponed. They chose the third option, and I am told the boycott option got the least votes. They will push for this with more protests.
I can understand there is concern with legitimizing what has become an illegitimate process, but I expect campaigning for no will be the only recourse left if protests, strikes, legal maneuvers and getting the backing of judges and other constituencies involved in the referendum's administration does not work. A postponement of the referendum (not a cancellation) is what makes the most sense here, and if Morsi was not stubbornly stuck on an insane process he started he could do that easily without losing face.
And the New York Times gets it wrong again here in a story titled "Opponents of Egypt’s Leader Call for Boycott of Charter Vote".