US presidents often seem like a reaction to the one before.
Calm, intellectual Barack Obama was an opposite of George W. Bush with his cowboy persona. Showman Donald Trump was an opposite of Obama.
Trump is such an unusual character that the entire top cast of the Democratic presidential primary would qualify as his opposites.
Trump at 72 is old for a modern-era president. He lacks a political, public service background. He's known as a businessman and reality TV star. In office, he has pushed the boundaries of norms and traditions of governing.
In contrast, of the about 10 leading contenders to compete against the President next year, several are senators. They include Bernie Sanders, Elizabeth Warren, Kamala Harris, Cory Booker and Amy Klobuchar. There's former Vice-President, Joe Biden. Beto O'Rourke is a former congressman.
Several candidates are from minority groups: Harris, Booker, Julian Castro and Andrew Yang. Mayor Pete Buttigieg is gay and a military veteran. Harris and Klobuchar are former prosecutors. Booker and Buttigieg are former Rhodes Scholars.
Biden, 76, and Sanders, 77, are older than Trump. Buttigieg is the youngest at 37.
Faced with picking from this all-sorts collection, Democratic voters have adopted a wait and see approach. Most of the leading lights have polled in the single digits. That could be a reflection of uncertainty among Democrats about what to prioritise. Is electability the most important factor? Is their main task simply to defeat Trump? How much change is required beyond that and how strong a change agent is needed? With several strong women in the field, why are two men in their 70s at the top?
But Biden's candidacy is now providing a few hints.
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Donald Trump's words push race to fore of 2020 campaign
He is now clearly the early front-runner, raising US$6.3 million in his first 24 hours of campaigning, the most of the bunch. In a Morning Consult poll on Tuesday, he jumped 6 points in a week to 36 per cent. Yesterday, a Quinnipiac poll had him at 38 per cent with Warren next best at 12 per cent.
Compared to his rivals, the former Vice-President is taking a more direct approach to Trump, drawing the President into verbal scraps. Other 2020 contenders are following the party's successful strategy in last year's Midterms of focusing on issues and policies of concern to voters such as healthcare.
On Tuesday, Biden held his first campaign rally in Pittsburgh, Pennsylvania, a key swing state with the working-class voters Democrats need for victory in 2020. Rightly or wrongly, Biden is seen as the best candidate to bring them back to the fold after they gave Trump a go in 2016.
Biden knows his strongest suit is to create a direct comparison in strength and status with Trump while highlighting his own likeability and talking a positive, empathic message.
"We have to choose hope over fear, unity over division and maybe, most importantly, truth over lies," Biden said.
It's designed for voters who want the US economy to stay on course but wish to unplug from the Trump drama.
For primary voters, nostalgic for a return to political "normality", and what appear a safe pair of hands, Biden might seem the answer.