★horn in
LONGMAN's definition⇒to interrupt or try to take part in something when you are not wanted [+on] (ex.)Several companies have tried to horn in on the market for computer chips like Intel's.
CONCISE OXFORD's definition⇒(horn in)＜informal＞ interfere
McCain was seriously horning in on Democratic candidate Se. Barack Obama's message of change.
マケイン氏は、（米大統領選）民主党候補のオバマ上院議員が発する変革の呼びかけに、（自身も「変革者」の看板を揚げ)本気で割り込んだ。