Booker joins a field already crowded with his Senate colleagues -- Sens. Kamala Harris (California), Kirsten Gillibrand (New York) and Elizabeth Warren (Massachusetts) are already in or expected to be in soon -- and with others like Sens. Amy Klobuchar (Minnesota) and Sherrod Brown (Ohio) also considering bids.

For Booker, his presidential bid is the culmination of a career -- despite the fact that he is not yet 50 -- that has long been on this trajectory. Since he emerged into the national spotlight as a crusading reformer taking on the established black power structure in Newark, New Jersey, in the early 2000s -- if you have not watched the documentary "Street Fight" about his 2002 run for mayor , you must -- through his election to the Senate in 2013, Booker has long carried the tag of "rising star who will run for president." (Admittedly, that wouldn't exactly fit on a name tag.)

And now, he's running. He enters the race with real strengths -- and some major questions surrounding him. Here are five things to think about Booker's candidacy.

1. He's the best, most natural speaker in the race

One of the ways to stand out in a very crowded presidential field is to have a skill no one else has. Booker has that in his emotional and charismatic speaking style. (If you need proof, watch his speech at the 2016 Democratic National Convention .) While Booker has -- and will -- face critics who hit him for his alleged theatrics, the truth is that Booker's speaking ability will light up rooms in Iowa, New Hampshire and South Carolina. And likely help him stand out in debates while standing beside nine other 2020 politicians. He is also likely to benefit in the eyes of some Democratic base voters from the (overly facile) comparison to the last charismatic African-American speaker in national politics: Barack Obama.

2. He has a solid core of longtime aides around him

Because Booker has been seen as someone who will run for president since, basically, 2002, he's attracted a large and talented group of senior aides who have been with him for years and who know him and his style well. Addisu Demissie, Booker's presidential campaign manager, managed his 2013 Senate bid. Matt Klapper, a senior adviser to the campaign, has been Booker's Senate chief of staff for years. Mo Butler was Booker's chief of staff when he was mayor of Newark and will be a senior strategist on the presidential campaign. It's hard to overestimate how important it is for a candidate running for national office to be surrounded by people a) who he trusts and b) who can say "no" to him. In the course of a two-year presidential campaign, you will say and do dumb things -- or at least think dumb things. You need people who can stand up to you and tell you not to say and do those things.

3. He's a tireless campaigner

Running for president is an absolute grind. You have to wake up a lot of mornings in some nondescript hotel room in Iowa or New Hampshire, with the thermometer hovering in the single digits and snow falling and spend the next 18 hours acting as though every person you meet is the single greatest person you have ever met. That's very, very difficult. But Booker's 2018 schedule -- in which he campaigned like crazy for candidates all over the country -- suggests he has the energy to do it and the understanding of what it takes. Plus, at 49, he will be nearly three decades younger than some of the other candidates in the race, which should help him manage the grind a bit better.

4. He's largely unproven as a candidate

Booker has never really faced a deep dive into his background from a well-funded opponent. And his 2013 special election victory to the Senate raised some questions about whether he could stand up to that sort of scrutiny. There was the odd direct messages on Twitter with a stripper from Oregon that were never fully explained. And the mysterious figure of "T-Bone," a drug dealer who Booker said once threatened to kill him. The problem was that when reporters went looking for T-Bone, no one could find him . And although Booker had repeatedly spoken of the man as though he was a real-live person, it appears as though there is no actual T-Bone. '"He is an archetype of so many people that are out there," Booker said in 2007 . "He is 1,000% a real person." Which, like, still leaves things cloudy. Is T-Bone a real person? Or an archetype? Booker doesn't lose a presidential race based on this question. But it does suggest he has some work to do on his back story. And fast.

5. His record isn't perfect for liberals

CORRECTION: This story has been updated to correct the year of Booker's "archetype" quote.