Sentence:  Mr Obama's problems were partly structural. An <WSD>incumbent</WSD> must defend the realities and compromises of government, while a challenger is freer to promise the earth, details to follow. Mr Obama's odd solution was to play both <WSD>incumbent</WSD> and challenger, jumping from a defence of his record to indignation at such ills as over-crowded classrooms and tax breaks for big oil companies. 
Meaning of the word marked with <WSD> tags:  The current holder of an office, such as ecclesiastical benefice or an elected office.