Definitely need to be a US citizen for it to be considered treason, but the argument is really about whether he was born as a US citizen or not.

For all intents and purposes he is a US citizen now and has held public office and can be considered as a traitor to the US.

Don't let the left bamboozle everyone with their shaping of the issue, it has always been about Obama having the qualification to hold the presidency due to him being born overseas, not wether he was a US citizen or not.