It may be the largest rally in support of science ever. Hundreds of thousands of people have joined the Facebook group for the upcoming March for Science, and tens of thousands have offered to volunteer. Beyond a march in Washington, more than 400 cities worldwide will host simultaneous events on April 22 to repudiate science policies of the new White House and Congress.

Yet for all the excitement, STAT has found, plans for the march are plagued by infighting among organizers, attacks from outside scientists who don’t feel their interests are fairly represented, and operational disputes. Tensions have become so pronounced that some organizers have quit and many scientists have pledged not to attend.