Election season is getting under way and the race to become the Democratic challenger to Donald Trump is hotting up.

Last summer, there were nearly 30 serious candidates vying for the attention of the party's supporters, but only two are still standing.

Here's a brief guide to who they are, with some analysis on each of them from the BBC's Anthony Zurcher.

Who are they? What are their key issues? What's their secret weapon against President Trump? We've got it all covered.

Who will take on Trump in 2020? Joe Biden Obama's VP Veteran politician who was a Delaware senator for more than three decades before becoming Barack Obama's vice-president in 2009. Key issues: Rebuilding the middle class; investing in federal infrastructure; tuition-free public universities. Anthony's take: For the better part of 2019, campaign-watchers in the media and the political world were waiting for the former vice-president's stumbles and bumbles to catch up with him. Entire candidacies (Cory Booker, Kamala Harris and others) were built around the premise that the early frontrunner was poised for a fall. That hasn't ended up happening. While other candidates have surged and faded, Biden keeps on keeping on. His support has been doggedly stable, drawing largely from elderly, moderate and black voters. He doesn't have as much money or draw crowds as big as some of his rivals - and that could catch up with him in the end. He hasn't faded, but he hasn’t surged either - and the only way that may happen is if he becomes the safe harbour candidate for voters concerned about more radical alternatives. But even if the road gets rocky, Biden has proven he has staying power. Secret weapon: Comfortability. Like an old pair of jeans, Biden is a known, unthreatening quantity. After four years of Donald Trump, that could be the ticket to victory.

Bernie Sanders Mr Anti-Establishment Vermont senator and self-proclaimed "Democratic socialist" who came close to the nomination in 2016. Key issues: Medicare-for-All universal healthcare coverage; raising taxes on the wealthiest Americans; upping the minimum wage. Anthony's take: The big question hanging over Sanders' 2020 presidential campaign was whether it would be able to recapture the magic of his 2016 effort. For a while, it was no sure thing. The 78-year-old candidate had a heart attack in October, raising concerns about his age. He was competing in a much more crowded field, which featured another liberal heavyweight, Elizabeth Warren. His polling support ebbed and flowed for much of 2019. By early 2020, however, he had re-established himself again as the progressive standard-bearer, whose sometimes gruff and always consistent message about income inequality and corporate rapaciousness lent him an air of that much-sought-after modern political quality - authenticity. To win the nomination he’ll have to either count on a field where the moderate Democratic vote stays divided or ensure that his base reflects the diversity of the Democratic party - all while deflecting what will be a fierce effort by some in the party to derail his campaign. Secret weapon: Loyalty. More than any other Democratic candidate, Sanders has a core group of supporters who have stuck with him through thick and thin. They despaired when he lost in 2016, and have been planning his political revival ever since. Now their time has come.

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The race so far

Although the field has now been whittled down to a couple of contenders, at one point it had swelled to nearly 30 Democrats.

Former congressman John Delaney began his campaign back in the summer of 2017 and was joined a couple of months later by Andrew Yang. After two and a half years of campaigning, Delaney admitted defeat and withdrew in January. Yang dropped out after getting just 1% of votes in Iowa and 3% in New Hampshire.

Others, like Michael Bloomberg, left it late to get involved. His strategy was to focus on states that voted on Super Tuesday, spending huge amounts of his personal wealth on ad campaigns. Ultimately, he came away with just a handful of delegates before promptly quitting the race and endorsing Joe Biden.

Polls point to clear top tier

Joe Biden was the accepted frontrunner in this race throughout 2019. After serving as Barack Obama's vice-president for eight years, he had strong name recognition and held a clear lead in national polls.

His numbers dipped at the start of 2020 though and Bernie Sanders overtook him in the RealClearPolitics national average after a strong showing in the Iowa caucuses. Biden, however, won by a big margin in South Carolina, which helped boost his poll numbers before he then swept the South on Super Tuesday.

There's a lot of drama to come

Although there is usually a clear winner much sooner, the race officially ends in July at the Democratic National Convention where the candidates with the highest number of delegates becomes the party's presidential nominee.

Words: Anthony Zurcher, Mike Hills. Charts: Mike Hills. Development: Felix Stephenson, Alexander Ivanov, Steven Connor. Design: Debie Loizou.