Robert F. Kennedy Jr. is threatening legal action against Nevada over his petition to appear on the ballot as an independent candidate, his campaign said Monday, after CBS News reported that the signatures he had gathered could be invalidbecause his petition did not include a vice presidential candidate.The Kennedy campaign claimed that the Democratic Party invented a new rule to invalidate his Nevada signatures. But Nevada’s requirement for a vice presidential candidate to be named in an independent candidate’s petition has been on the books since 1993.