Does a militant wing help or hurt the moderates standing next to it?

The radical flank effect is the effect, positive or negative, that radical activists have on the more moderate activists in the same cause. So whether a militant wing helps or hurts is a question the evidence answers both ways.

The case for help starts with Herbert Haines, who coined the term. In 1984 he found that moderate Black organisations saw increased, not decreased, funding as a radical Black movement emerged. He argued against the view that militants created a white backlash against the moderate civil-rights movement, saying the turmoil they created was indispensable to black progress. One mechanism: radicals shift the boundaries of discourse, making moderates look more reasonable, as radical feminists of the 1960s and 70s made reformist women's groups seem mild. Another: radicals create crises that authorities resolve by making concessions to the moderates. And armed radical flanks sometimes protect nonviolent activists: in 1964 the Deacons for Defense and Justice guarded a pacifist CORE chapter in Louisiana and later protected Martin Luther King Jr. during the March Against Fear.

The case for harm is that radicals may discredit a movement and make it harder for moderates to work with third parties. Studies of civil resistance have typically found that violence by a movement makes state repression seem legitimate. It is difficult to tell without hindsight which effect will win, and scholars have proposed that radicalism at the peak of activism, before concessions are won, is more likely to help than radicalism after a movement declines.