The first Democratic caucus of the 2020 U.S. presidential race is still six months away, but this week's debates in Michigan could be make or break for some in the crowded field of 20 candidates vying to be the presidential nominee.

For the four candidates leading the race — former vice-president Joe Biden, California Sen. Kamala Harris, Massachusetts Sen. Elizabeth Warren and Vermont Sen. Bernie Sanders, all polling at over 10 per cent — the debates are a chance to take a top-tier rival down a rung or two.

For those at risk of dropping below the two per cent polling threshold necessary to qualify for the next debate in September, it's a fight for survival.

The candidates have been divided into two groups of 10, with Warren, Sanders and a number of mostly centrist Democrats, including former Texas congressman Beto O'Rourke, South Bend, Ind., Mayor Pete Buttigieg and Minnesota Sen. Amy Klobuchar, facing off Tuesday — the first of two nights of debate at the Fox Theatre in Detroit.

The following night will see Biden, Harris and a mix of left-leaning and outlier candidates, like New York Sen. Kirsten Gillibrand and Hawaii Rep. Tulsi Gabbard, and up-and-comers who, although trailing far behind, have been building some name recognition, such as Julián Castro, a former housing secretary in the Obama administration, and tech entrepreneur Andrew Yang.

1st matchup for Sanders, Warren

The first night, all eyes will be on Warren, 70, and Sanders, 77, who haven't yet had the opportunity to square off and are ideologically aligned on the far left on issues such as Medicare for all, increasing taxes on the wealthy and student debt forgiveness.

Sen. Bernie Sanders made a trip to a pharmacy in Windsor, Ont., this past weekend to highlight his single-payer universal health-care plan ahead of the debates. (Rebecca Cook/Reuters)

While Sanders still has a strong base of support among young Democrats, he has been losing momentum and votes to Biden and Warren.

"[Warren] has spent the last year peeling away his liberal support and his super-liberal support," said veteran Republican strategist Mike Murphy. "So, Bernie needs something to happen because he's fading away slowly. So he may go on the offence."

Sen. Elizabeth Warren has the advantage over Sanders going into the first night of the debates, which will be the first time the two candidates, who have similar policies, go head to head. (Justin Sullivan/Getty Images)

Night 2 will be all about the Biden-Harris rematch. Biden — the far-and-away frontrunner, with support above 30 per cent, about twice that of Warren and Sanders — needs to make up for his poor performance in the last debate, where he was caught off guard by Harris and grilled on his stance on school desegregation in the 1970s.

WATCH | Harris questions Biden about his opposition to some federally mandated measures to desegregate schools in the 1970s:

California Sen. Kamala Harris challenges former vice-president Joe Biden on his previous positions on segregation. 4:07

"Going into the last debate, Joe Biden was probably advised to stay above the fray. I think that turned out to be the wrong advice," said Robert Shrum, a former campaign strategist who advised Democratic presidential candidates Al Gore and John Kerry in the 2000s and who now runs the Center for the Political Future at the University of South California.

"I don't think he'll initiate attacks, but I think he will certainly counterattack, and he will certainly vigorously defend himself."

Harris will need an 'Act 2'

Although it wasn't on display in June, Biden, 76, has been a feisty debater in the past and has since been vocal in defending his civil rights record, so Harris, 54, won't be able to coast on her success.

WATCH | Biden gave a more fiery performance in the 2012 vice-presidential debate against Paul Ryan:

"What is her Act 2? Is she going to go after Biden again? And what's she going to go after him on? I think there'll be a lot of drama in these two debates," Shrum said.

Harris has already tried to get out in front of her perceived weakness, namely flip-flopping on some key policy issues, such as health care. She released a platform Monday that outlines a plan to preserve private insurance and phase in an expanded public option over 10 years.

Sen. Kamala Harris addresses the audience at the annual convention of the National Association for the Advancement of Colored People in Detroit last week. At a time of heightened racial rhetoric, all of the leading candidates in the Democratic race are courting the crucial African American vote. (Rebecca Cook/Reuters)

Muscling in between Biden and Harris will be Cory Booker, the 50-year-old African American senator from New Jersey who is trailing far behind but has recently gone on the offensive, challenging Biden's record on criminal justice and race.

Race was going to figure in this week's matchups even before U.S. President Donald Trump on Sunday inflamed the already heated debate he provoked two weeks ago by criticizing the majority-black district of Elijah Cummings, a Democratic congressman who has been one of his most prominent critics on the Russia investigation.

Sen. Cory Booker is only polling at around two per cent, but inserted himself into the Biden-Harris standoff by also questioning Biden's track record on race. (Brian Snyder/Reuters)

"I think it's a moral and political imperative for Democratic candidates to talk about the race issue after what Trump has done, and I think they will," Shrum said.

Gun control will likely also come up in light of the fatal shooting Sunday at a garlic festival in California.

'They need Biden to stumble'

There is broad agreement on those issues across the slate of candidates, but there might be opportunities for those clumped near the bottom of the polls to chip away at the front of the pack, said Ed Kilgore, a political analyst who has advised Democratic lawmakers in Georgia and runs the Democratic Strategist, an online policy forum.

Someone like O'Rourke, 46, who clashed with Castro on immigration in the first debate and has lost ground to Buttigieg, who has roughly three times the support and more than double the fundraising, could try to make inroads.

Beto O'Rourke is expected to try to regain some of the support he has lost since he first entered the race in March on the debate stage Tuesday night. (Brian Snyder/Reuters)

"These candidates that are not doing that well, but that have a pretty sharp ideological profile — whether it's left or centre — their temptation will be to go after one of the headliners who's from a different wing of the party rather than each other," Kilgore said.

The line of attack has to be chosen carefully, however. Age, which Eric Swalwell tried unsuccessfully to highlight in his exchange with Biden in the last debate, and which 37-year-old Buttigieg has incorporated into his campaign message of "generational change," is not likely to get traction in the current climate, said Shrum.

"Democrats are dominated more than anything else by the desire to beat Donald Trump, and they don't care whether it's someone 75 or someone 45," he said.

Pete Buttigieg, mayor of South Bend, Ind., is among the strongest fundraisers of the 20 candidates vying to be the Democratic nominee but is polling only at around six per cent. (Brian Snyder/Reuters)

Rumblings that some of the lower-tier candidates are digging deep into Biden's record could be a sign they will try to undercut the former vice-president, whose moderate, return to pre-Trump status quo agenda some see as the best hope to beat Trump in 2020.

"They need Biden to stumble because they need room to be the centre-left candidate," Shrum said.

Frontrunner Joe Biden, former vice-president in the Obama administration, is seen by some as the safe, establishment candidate best positioned to take on Trump. (Justin Sullivan/Getty Images)

A newcomer who could snag some media spotlight is Montana's Steve Bullock, 53. As a Democratic governor of a red state that Trump won by 20 points in 2016, he's in a position to set himself apart, said Murphy.

"He's very different in tone and vibe from what they've been seeing with the northeastern liberals," said Murphy. "So he'll have the advantage if the moderators give him an opportunity."

Montana Gov. Steve Bullock is a new arrival on the debate stage in this second round. (Steve Pope/Getty Images)

A lot could happen between now and 1st primary

But it will be hard for the so-called one-percenters to make the leap needed to break out of the margins, especially given the tight, two-hour format. Yang, for example, who has garnered some buzz for his proposal to tax Amazon and implement a basic monthly income of $1,000 to offset jobs lost to automation, got less than three minutes to speak in the last debate.

That doesn't leave much opportunity for generating memorable one-liners or viral moments.

"However much debate prep you do, a lot of it is up to the moderators and just the ebb and flow of the debate," Kilgore said. "With that many people on the stage, you're not going to get second and third chances."

Still, going in prepared allows candidates to seize opportunities when they arise, said Peter Hart, an expert in public opinion polling who runs the NBC News/Wall Street Journal poll.

Even iconic moments, such as Lloyd Bentsen's "Senator, you're no Jack Kennedy" riposte to Dan Quayle in the 1988 vice-presidential debate, are rarely entirely spontaneous, he said.

WATCH | 'Senator, you're no Jack Kennedy': Lloyd Bentsen takes issue with Dan Quayle comparing himself to John F. Kennedy in a 1988 vice-presidential debate:

"How do you get noticed? Sometimes it can be something as simple as taking on another candidate. In other times, it is revealing something about yourself that voters find is part of the inner soul or a sense of the character of the candidate," Hart said.

"There are a lot of routes to what we call victory in a debate."

At the end of the day, the debates are just one in a long string of events between now and Feb. 3, 2020, when Democrats vote in their first caucus in Iowa and apportion some of the delegates who will select the presidential nominee at a party convention in July.

"Looking at the polls today is like looking at a galaxy about as far into space as you can imagine," Hart said.

Those breathlessly covering every spike and slide in the polls would do well to recall that in July 2015, 15 months out from Trump's 2016 election win, Hillary Clinton was ahead of Bernie Sanders 75 per cent to 15 per cent, he said.

"In the same poll on the Republican side, Donald Trump was at one per cent, having just announced, and Ted Cruz was at four per cent, and the top five candidates were dividing 72 per cent of the vote."