During the third Democratic presidential debate, Hillary Clinton proposed a "Manhattan-like Project" to foster cooperation between tech companies and the government. The Manhattan Project, of course, was the top-secret U.S. military project during World War II that resulted in the creation of the atomic bomb. Clinton made the comment during a conversation about data encryption; her point was that, when the Manhattan Project was active during the 1940s, it was a high-water mark for cooperation between the government and private technology companies. That's all well and good — but was the nuclear weaponry comparison really the best way to drive this point home?
Many think that no, it wasn't. To be entirely clear, Clinton was most certainly not proposing that the United States start upping its stockpile of nuclear weapons, or that the government launch another secret operation to develop a massively destructive and hitherto-unseen weapon. She was simply saying that in order to combat things like cyberattacks, ISIS recruiting through social media, and other threats that are online in nature, the government should launch some sort of project to compel the tech industry and the government to work together effectively.
Again, that's a perfectly fine proposition. But many felt that using the Manhattan Project as the point of reference was, well, not appropriate.
In general, it may be best for candidates to avoid invoking nuclear war when at all possible.