LAS VEGAS - The Republican presidential debates have become appointment television. Tens of millions of Americans have tuned in to see if Donald Trump might whack his unwieldy field of opponents - pooh-poohing Jeb Bush's energy level, for example, or disparaging Carly Fiorina's appearance.

So, what happens if the biggest fireworks in the first Democratic debate at the Wynn Las Vegas casino hotel on Tuesday night are over which candidates opposed the Keystone XL pipeline first, which would resurrect the Glass-Steagall Act, or how each would pay for his or her higher-education overhauls?

Democrats expect their debate to be substantive and to set the course for an unexpectedly contentious nominating contest. Americans are either going to find a pleasing contrast to the rip-roaring show the GOP has put on - or they will be bored senseless.

"Let's be honest: Donald Trump truly is a ratings machine. Twenty-three million people did not tune in to see Marco Rubio," said Paul Begala, a Democratic strategist and analyst on CNN, which is airing Tuesday's debate. "So, unless the Democrats can talk one of the Kardashians into running, don't expect the Democrats' ratings to approach the Republicans'."

For good or bad, Republicans have engaged a massive swath of the country with their first two debates, which were watched by 25 million and 23 million people, respectively. (By comparison, the highest-rated Democratic presidential debate in 2008, when Barack Obama and Hillary Rodham Clinton, locked in a battle royale, drew 10.7 million viewers.) Even if some voters only tuned in to witness the spectacle that is Trump's front-running candidacy, they now are familiar with many of the other GOP candidates.

A test for Democrats is whether Clinton, a former secretary of state and first lady, and a cast of four challengers, led by liberal insurgent Sen. Bernie Sanders (I., Vt.), can generate the same level of interest in their primary campaign.

In the run-up to Tuesday's debate, there are signs that they could. On Sunday, there were 124,812 mentions of the Democratic candidates on social media, television and in newspapers. By comparison, there were 377,223 mentions of the Republican candidates two days out from their Sept. 16 debate, according to an analysis by Zignal Labs, the Washington Post's analytics partner.

However, the Republican statistic is larger in part because it included many more candidates, according to Zignal. Clinton and Sanders consistently have a larger social media presence than any GOP candidate besides Trump.

As they prepare for their first face-off, Clinton and Sanders have signaled that they will wear velvet gloves. Each plans to focus on his or her own policy proposals and backgrounds, drawing comparisons with each other's wherever appropriate but avoiding the kind of direct, personal attacks that have been so prominent in the Republican race.

None of the Democratic candidates is Trump's equal in bombast and showmanship. And unlike many of the other Republican candidates, the Democrats are downright predictable.

Sanders will probably bring the most passion, as he has all summer and fall, drawing massive crowds across the country. Clinton, who earned a reputation as a steady and skilled debater in her 2008 campaign, is by her nature cautious and likely will display more competence than entertainment value.

Two lesser-known candidates, former Virginia Sen. James Webb and former Rhode Island Sen. Lincoln Chafee, tend to be understated.

The wild-card could be former Maryland governor Martin O'Malley, who is struggling to break out of the low single-digits in polls and sees the fall debates as make-or-break opportunities. On the campaign trail, he has fired shots at Clinton, and he could do so on the debate stage.

Still, Anderson Cooper, the CNN anchor and moderator of Tuesday's debate, said he does not anticipate the Democratic candidates will be willing to criticize each other too much. He is preparing for a different kind of debate than the Republicans have held.

"This is a serious debate. They want to talk about the issues," Cooper said Sunday on CNN's Reliable Sources.

CNN chief political correspondent Dana Bash and CNN en Español anchor Juan Carlos Lopez will join Cooper as questioners. CNN anchor Don Lemon also will ask questions submitted through Facebook, which is co-sponsoring the debate.

The debate, at the Las Vegas luxury resort will last two hours and begin at 8:30 p.m. ET. Though five candidates are expected to be on stage, should Vice President Biden announce his entrance into the race before Tuesday night he is pre-qualified to appear at the debate. His advisers have said he will not attend, although CNN has a sixth podium at the ready.

Harvard law professor Lawrence Lessig, who says he has raised more than $1 million from a devoted base of supporters but is polling at or below 1 percent, was not invited.