Caught off guard by his sudden surge, Pete Buttigieg's rivals are scrambling to find vulnerabilities and lines of attack that can be used against him, five officials with opposing Democratic primary campaigns and Republican political groups tell NBC News. The situation is different than with Bernie Sanders, Elizabeth Warren and Joe Biden, Democrats who have long been on the national scene and were widely expected to run for president. Potential rivals and GOP campaign groups have spent years hunting for dirt — known in political parlance as "opposition research" — that could be deployed against them. Major political groups had...