Eighteen members of the: Republican Party have served as president, governing for 88 years in total. Four served two full terms and five served one full term. Three were elected twice but either died or were assassinated (Abraham Lincoln and William McKinley) or resigned (Richard Nixon) during their second term. Two were elected once but died before completing their first term (James A. Garfield and Warren G. Harding). Four succeeded to the presidency following the death or resignation of their predecessor: two (Theodore Roosevelt and Calvin Coolidge) were subsequently elected to a full term; two (Chester A. Arthur and Gerald Ford) were not.
Fifteen members of the Democratic Party have served as president, governing for a total of 85 years. Five served at least two full terms (Franklin Roosevelt was elected four times; he served three full terms and died during his fourth) and five served one full term. Three succeeded to the presidency after the death of their predecessor (of which two were subsequently elected to a full term [i.e., Harry Truman and Lyndon Johnson] ) and one, Andrew Johnson, did not seek election). John F. Kennedy was assassinated before his first term was finished and Barack Obama is currently in office.
John Quincy Adams (1825-1829) was the president when the Democratic-Republican Party split. He led the National Republicans (aka: Anti-Jacksonians) along with Henry Clay (who later founded the Whig Party).
The National Union Party was the name used by the Republican Party for the national ticket in the 1864 presidential election. Both Abraham Lincoln and Andrew Johnson associated with the National Union Party during the 1864 election and Andrew Johnson remained a member until he joined the Democratic Party at the end of his second term. Lincoln and Johnson served as President while members of the National Union party for 4 years.
^ abLincoln served his first term (from 1861-1865) as a member of the Republican Party, but ran for his second term as a member of the National Union Party and served only a short part of his second term in 1865 before being assassinated.
^ abAndrew Johnson did not identify with the two main parties of Democrats and Republicans, though he did try for the Democratic nomination in 1868, and so while President he attempted to build a party of loyalists under the National Union label. Asked in 1868 why he did not become a Democrat, he said, "It is true I am asked why don't I join the Democratic Party. Why don't they join me ... if I have administered the office of president so well?" This failure to build a new National Union Party made Johnson effectively an independent.
^ abAlthough John Tyler was elected vice president on the Whig ticket, his policies soon proved to be opposed to most of the Whig agenda, and he was expelled from the party in 1841, a few months after taking office as President.