Nationally-coordinated action can occur without relying on the compulsion of hardline democratic centralism.
In such an environment, coordinated activity happens when someone persuades multiple activists or local bases to take up the same project in multiple locations. These forces can then communicate, comparing notes on how their attempts are proceeding.
This can totally “start from the bottom” of an organization. It does not require leadership sanction. It does not even necessarily require majority sanction. It is simply an idea and a practice that may be contagious, to greater or lesser degree.
Given that local bases often have their own campaigns unique to their own contexts, what may often emerge is not necessarily one coordinated national campaign, but instead a similar style of approaching these different situations, and the comparing of notes about this style of organizing (applied in different ways).
Nonetheless, someone may still end up making a compelling argument within an organization that the entire organization should launch some kind of national campaign by spearheading it themselves, and using their collective, coordinated weight to do this. Self-initiated projects are inherently audacious and risky (though these are the last things that should be daunting to a socialist – better to fear pointless tedium).
Such true, nationally-coordinated activity can be extremely powerful. In bureaucratic groups, it is easy to pull off, because the group culture is compliant uniformity. Everyone does the same thing everywhere as a matter of course (though any advantages in coordination are countervailed by these groups having less flexibility for connecting to local activity). In democratic groups, especially ones with a federal, non-compulsory structure, it is much harder to actually make this happen. But if a person can make a strong case for why material conditions, untapped sentiment, and other factors align to make a certain approach into a huge opportunity, then an enthusiasm can sweep the organization and the project becomes more-or-less national.