Is there a baseline of requirements, such as $x in the war chest, number of endorsements, or something similar?
Not in 2017 for 2020. At this time in 2013, people expected Hillary Clinton to run but weren't sure. There were some concerns over her health that had caused her to leave the State department. Some people wanted Bernie Sanders to run, but he hadn't made a decision.
As a practical matter, most candidates do not register for the presidential election until the year before it. That means that they can't raise money as a presidential candidate. Fundraising for other seats tops out around $50 million. Candidates for president are raising ten times that. So funds raised before announcing are not significant.
Endorsements are similar. Until the announcement is official, candidates can't really rely on endorsements.
The typical candidates running now will be trying to raise their profiles. This could be any of the following:
- Taking public positions that get them in front of the television (this is usually why the media speculates that someone is going to run).
- Specifically, voting against every Donald Trump appointee.
- Testifying against a fellow Senator's cabinet appointment for the first time in history.
- Supporting other candidates, to make other politicians reliant on them. This often leads to endorsements later.
- Contributing funds to other campaigns.
- Campaigning with other candidates.
- Joint fundraisers with other candidates.
- Endorsing other candidates in their own races.
- Giving advice to other candidates.
- Building a campaign organization.
- Giving speeches to organizations outside one's district. This could be something big like CPAC or something smaller.
Of course, politicians may do these same things for other reasons, like trying for leadership positions in Congress. Or just winning reelection. Or even just because they are the right thing.
The most qualifying job for president is governor. Note that George W. Bush, Bill Clinton, Ronald Reagan, and Jimmy Carter were all governors. George H. W. Bush and Richard Nixon were Vice-President. John F. Kennedy was a Senator. Dwight Eisenhower was a general. Gerald Ford, Lyndon B. Johnson, and Harry S Truman all assumed the presidency from the vice-presidency.
Chances are that the 2020 Democratic candidates for president are all either governors or Senators. Martin O'Malley is a possible exception--he's a retired governor. It's also conceivable that someone with a background more like Trump's might run. This could be someone like Oprah Winfrey or Mark Cuban. Michael Bloomberg is probably too old.