Question: Under which US president was a politician in opposition of John G Schmitz?
SPARQL:
SELECT DISTINCT ?uri WHERE { ?x <http://dbpedia.org/property/opponent> <http://dbpedia.org/resource/John_G._Schmitz> . ?x <http://dbpedia.org/ontology/incumbent> ?uri  . ?x <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/Politician>}