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).
2008 United States elections
Senator Barack Obama of Illinois was the Democratic nominee, and Senator John McCain of Arizona was the Republican nominee. Incumbent President George W. Bush was ineligible for re-election per the 22nd Amendment, which limits a president to two terms, and incumbent Vice President Dick Cheney declined to run for the office.