General Discussion
In reply to the discussion: Elizabeth Warren Comes Down Hard Against Global Warming, Separates Herself From Hillary Clinton [View all]karynnj
(60,944 posts)by Obama. (The reason for pre approved is that if he ever made an offer, it was accepted - and Obama rescinded it, he would lose credibility as a negotiator.)
In addition, it would be wrong if he wrote an op-ed on some issue taking a position against Obama's position. As he himself said - I think to the London embassy staff - he, like everyone at the embassy works for Obama - and there is no more "I", but "we". (rough paraphrase)
However, a Secretary of State CAN advise the President and privately disagree. A very significant example is that George Marshall, one of the best SoS the US had, advised and lobbied the President not to support the partition that led to Israel - the President instead led the US to be one of the first countries to recognize Israel. (Consider that VP Biden disagreed with Obama on the surge in Afghanistan - an issue at least as important. We know Clinton wanted a larger surge.)
Kerry, as a Senator, voted against Keystone - so we know his personal opinion. He also has avoided accepting the flawed state department study - giving time for more negatives to come out. The EPA has already advised Obama against the pipeline. The actual approval really rests with Obama, who could approve it with or without Kerry.
I doubt that Kerry would resign over the issue because he then gives up the chance to continue working to make the world more peaceful.