Washington (CNN) They've studied the highs and lows of his campaign. They've hired some of his former team, elevating young aides who cut their teeth on his race. They've sought advice from him directly, hoping to absorb lessons from the last senator who won an unlikely bid for the presidency.

Every ambitious Democrat eyeing the White House wants to be the next Barack Obama.

The 44th president looms in the background over the early stages of the party's primary fight, with his candidacy providing a road map for a swelling field of candidates. He has met with most of them -- even a few who have yet to announce -- offering guidance from someone who has walked in their shoes.

He plans to stay on the sidelines in the Democratic primary contest, telling friends he believes "the torch should be passed." But he has taken calls and meetings from any prospective candidate who asked, eager to discuss challenges facing the country and what it's like to run for -- and serve in -- the Oval Office.

It's been 12 years since Obama formally announced his candidacy, standing before a cheering crowd at the Old State Capitol in Springfield, Illinois, where Abraham Lincoln began his political career. The 45-year-old junior senator was a rising star, but a long-shot presidential candidate.

"I recognize that there is a certain presumptuousness in this, a certain audacity to this announcement," Obama declared. "I know that I haven't spent a lot of time learning the ways of Washington, but I've been there long enough to know that the ways of Washington must change."

Victory was hardly preordained for the young senator, particularly with Hillary Clinton and other more experienced candidates in that campaign. But Obama's ultimate success is one explanation for the large and diverse field of 2020 candidates: Strike while the iron is hot; there's little upside to waiting.

To be sure, the parallels between the campaigns of 2008 and 2020 have their limits.

The party has shifted to the left, partly in response to a dissatisfaction among liberals at the record of the Obama administration. And the Democratic nominee will likely face President Donald Trump, rather than run for an open seat as Obama did in the waning years of the Bush administration, during the height of opposition to the Iraq War.

Yet the basic lessons of that contest still apply today: Capitalizing on the highs is often the easy part, while weathering the lows -- and rising above them -- poses the biggest test for any candidate.

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"Presidential races are gauntlets, and in a rough way they are supposed to be, where you simulate the kinds of pressures that people will face if they become president," said David Axelrod, an architect of the Obama campaign. "People get a chance to judge you in those moments, and we certainly had those moments."

How candidates grow over the course of a campaign and recover from their mistakes may be the most important test for them. Early conventional wisdom holds little currency, particularly in this unwieldy and wide-open Democratic nominating fight.

A poor performance in the first primary debate in South Carolina in April 2007 didn't doom Obama's candidacy, but it taught him that he had to work harder. He spent the next day explaining his lackluster performance, saying: "I was a little bit nervous. Not so much because it was my first presidential debate, but because we had a 60-second time limit on our answers and my wife says it takes me 60 seconds to clear my throat."

In fact, Obama struggled for months before finding his voice. But he also benefited from being underestimated by nearly all of his rivals, which is another telling lesson for the 2020 campaign.

The historic nature of his candidacy and a muscular fundraising operation kept him alive as he found his own footing, which will not be a luxury afforded to all the candidates in this crowded primary lineup.

"I did not know how he would handle the pressures of the race. We don't know that now about any of these candidates," said Axelrod, who is now a CNN contributor. "Some of them are going to surprise us on the upside; some of them will surprise us on the downside."

In the early handicapping of the Democratic field, Obama may have unique insight. He has held conversations -- some stretching more than an hour -- with most of the candidates.

While aides to Obama declined to discuss specifics of the calls, two potential candidates who have spoken to the former president told CNN they found the conversations helpful, particularly about ideas for how to win back voters who supported Obama in 2008 and 2012 but turned to Trump in 2016.

"President Obama wants to help Democrats win, and that is why he has been happy to speak privately with candidates and potential candidates seeking his guidance on the best way to lead the country," said Katie Hill, the president's communications director.

"President Obama's private counsel to candidates largely mirrors his public pronouncements that the most successful candidates will always show up, be present and make their case even in areas or in front of audiences they may not necessarily win, express views and positions that reflect their genuine beliefs and share a positive vision for the country true to their own personal story," Hill added.

Obama's pledge to not endorse a candidate in the primary could be complicated by whether his former vice president, Joe Biden, decides to join the race. Biden and Obama have not had a detailed conversation about his plans, two people familiar with the matter say, but are expected to speak about it before Biden makes a decision in the coming weeks.

Axelrod, who also served in the White House for the first two years of the Obama administration, described their relationship as "exceptionally close."

"Whether that would encourage President Obama to intervene in the race, I don't know. I doubt it," Axelrod said. "I think he's going to allow the candidates to make their own case. He believes in the process and believes the best candidate will emerge from that process."

Axelrod said Obama will be watching, along with the rest of the Democratic Party, to see who grows as a candidate and emerges as the strongest contender.

"He doesn't believe anyone should try and dictate who that nominate should be," Axelrod said. "In 2008, we entered as a long-shot candidate and the people of Iowa propelled Barack Obama forward. He's going to look to see who emerges from that process and from the process in other states, rather than trying to tip the scales in favor of any candidate."