Twenty-second Amendment to the United States Constitution
Some argue that the 22nd Amendment and 12th Amendment bar any two-term president from later serving as vice president as well as from succeeding to the presidency from any point in the presidential line of succession. Others contend that the original intent of the 12th Amendment concerns qualification for "service" (age, residence, and citizenship), while the 22nd Amendment, concerns qualifications for "election", and thus (strictly applying the text) a former two-term president is still eligible to "serve" as vice president (neither amendment restricts the number of times an individual can be elected to the vice presidency), and then succeed to the presidency to serve out the balance of the term (though prohibited from running for election to an additional term).
President of the United States
Presidents may use their predecessors as emissaries to deliver private messages to other nations or as official representatives of the United States to state funerals and other important foreign events. Richard Nixon made multiple foreign trips to countries including China and Russia and was lauded as an elder statesman. Jimmy Carter has become a global human rights campaigner, international arbiter, and election monitor, as well as a recipient of the Nobel Peace Prize. Bill Clinton has also worked as an informal ambassador, most recently in the negotiations that led to the release of two American journalists, Laura Ling and Euna Lee, from North Korea. Clinton has also been active politically since his presidential term ended, working with his wife Hillary on her 2008 and 2016 presidential bids and President Obama on his 2012 reelection campaign.