For nearly four years, federal prosecutors have tracked, charged and imprisoned hundreds of Donald Trump supporters who stormed the Capitol on Jan. 6, 2021. But what once seemed to be a shock-and-awe campaign of prosecutorial force — one the department has regularly characterized as the largest and most complex investigation in history — appears increasingly squeezed by legal challenges and political threats. Ongoing court battles threaten to erase or undermine some of the 1,500 cases...