I frequently see the argument that the 2020 Democratic presidential nominee could be a complete unknown, but historically that is very unlikely. Here are what recent presidential nominees were doing (and had done) four years before they were nominated. I'm excluding incumbent presidents who sought re-election. In almost every single instance, the nominee was either a current or former governor, senator, or cabinet official (including vp), or had just won election to one of those positions. The only exceptions were Trump and Eisenhower, two very well-known public figures
Trump ('16): A prominent businessman and tv figure who had gained political prominence with his questioning of Obama's birth. He was
mentioned as a potential independent candidate in 2012.
Clinton ('16): She was Secretary of State and had previously served in the Senate and as First Lady
Romney ('12): He was governor of Massachusetts from 2003 to 2007 and sought the presidential nomination in 2008
Obama ('08): He won election to the Senate in November 2004 and had been the keynote speaker at the 2004 DNC
McCain ('08): He had been in the Senate since 1987 and had sought the presidential nomination in 2000
Kerry ('04): He had been in the Senate since 1985
Bush ('00): He was governor of Texas since 1995 and the son of a former president
Gore ('00): He was VP since 1993 and had been a Senator
Dole ('96): He was the Senate Republican Leader
Clinton ('92): He had been Governor of Arkansas from since 1983
Dukakis ('88): He was Governor of Massachusetts from 1975 to 1979 and from 1983 to 1991
Bush ('88): VP since 1981
Mondale ('84): he was VP from 1977 to 1981 and a former Senator
Reagan ('80): He was Governor of California from 1967 to 1975 and had almost won the 1976 Republican presidential nomination
Carter ('76): He was Governor of Georgia from 1971 to 1975
Ford ('76): He had been House Minority Leader
McGovern ('72): He had been a Senator since 1963
Nixon ('68): Former VP and 1960 Republican nominee
Humphrey ('68): VP from 1965 to 1969
LBJ ('64): He was the vice president-elect and the Senate Majority Leader in 1960
Kennedy ('60): He was in the Senate from 1953 to 1961
Nixon ('60): He was VP from 1953 to 1961
Stevenson ('56): He had been governor of Illinois from 1949 to 1953
Stevenson ('52): He was elected Governor of Illinois in 1948
Eisenhower ('52): He was a distinguished general from WW2
Dewey ('48): Governor of New York since 1943 and the 1944 Republican presidential nominee
Truman ('48): He was a Senator and the VP-elect in 1944
