Your result
Portugal — Marriage
Possible
This is genuinely unsettled, and you should not be told otherwise.
The statute says nationality may be acquired by a declaration made during the marriage. Where the declaration was made before your spouse died, your position is good: the acquisition dates from the declaration, and what happens to the marriage afterwards does not undo it. If that is your situation, the file continues and the death certificate simply joins it.
Where the declaration was never made, the question is whether death ends the possibility in the same way a divorce does. The courts have addressed divorce after a declaration; they have not clearly addressed a declaration attempted after a death, and registry practice is not something we can state with confidence. This is a case to take proper advice on rather than to act on general information — and it is worth doing promptly, because if there is an argument to be made it will not improve with time.
Two things that are not in doubt. If you have children of the marriage, their claim through your late spouse is unaffected by the death, and if their nationality has not been registered, that should happen regardless of what happens to yours. And if you have lived in Portugal, the residence route runs independently of the marriage: seven years for nationals of Portuguese-speaking countries and of EU states, ten for everyone else.