More than two years have passed since the first presidential announcement, nearly $1 billion has been spent and numerous candidates have already come and gone. And yet the Democrats’ turbulent 2020 primary season officially begins Monday. By day’s end, tens of thousands of Iowa Democrats will have decided the results of their presidential caucus in the contest to challenge President Donald Trump. Democrats enter the first contest with uncertainty and deepening intraparty resentment. The top four candidates are Vermont Sen. Bernie Sanders, former Vice President Joe Biden, Massachusetts Sen. Elizabeth Warren and former South Bend, Indiana, Mayor Pete Buttigieg.