Tonight's fifth Republican debate will be on CNN. But don't worry, if you don't have cable, you'll still be able to tune in — an online livestream will be available at CNN.com. It has begun, so tune in now if you want to see it live.

Expect the stage to (still) be quite crowded. Nine candidates have qualified for the primetime debate segment: Donald Trump, Ted Cruz, Marco Rubio, Ben Carson, Jeb Bush, Chris Christie, John Kasich, Carly Fiorina, and Rand Paul. And the undercard will feature Mike Huckabee, Lindsey Graham, Rick Santorum, and George Pataki.

However, increasingly, a top tier of candidates has emerged in national polls. First, there's Trump, who remains in first place, as he has been for five months. Different pollsters have differed on exactly how big the billionaire's lead is — one recent poll put him just five points up, and another showed him with an astonishing 27-point lead. But it does seem clear that Trump's recent call to ban Muslims from entering the US — which will surely be discussed at the debate — hasn't hurt him among the GOP electorate so far.

Joining Trump in the top tier are Ted Cruz and Marco Rubio, who appear to be his two most dangerous rivals. Cruz has recently surged to first place in the first-in-the-nation Iowa caucuses and second nationally, and that's clearly set off some alarm bells in Trump's camp. Indeed, in the past few days, Trump has seized on a thin pretext to justify viciously attacking Cruz. So we'll see how the newfound tension between the two plays out on the debate stage.

Marco Rubio, meanwhile, has had less poll success than Cruz, but usually polls in the top three nationally, above other potential Republican establishment choices. Rubio seems to be the logical choice for Republican elites to fall behind, but they've been remarkably slow to do so so far, and it isn't totally clear why. Rubio, too, has seemed to see Cruz as a major threat, and has criticized him for being weak on national security for supporting surveillance reform.

Below that top tier is, well, everyone else — a bunch of candidates who haven't gotten any good news lately but who are desperately hoping something will change for them. That includes Ben Carson, whose support has sharply declined recently after controversies about his background and some odd statements he's made, as well as Jeb Bush, who is still hoping his Super PAC's massive spending will revive his fortunes at some point.

Though the debate will take place in Las Vegas, one subplot will be the battle for New Hampshire. Three candidates — Chris Christie, John Kasich, and Carly Fiorina — only actually qualified for the primetime segment because of their strength in New Hampshire polls, and are clearly staking their campaigns on the Granite State. Bush and Rubio, too, are hoping for a strong showing in New Hampshire. Unfortunately for them all, though, Donald Trump has led every poll in the state since June.

How to watch:

When: 8:30 pm Eastern tonight (is when "coverage begins")

Where: The Venetian Las Vegas

TV: CNN

Online: At CNN.com, a free live stream will be available

Update: Clarified that CNN has been vague about when exactly the debate, rather than its coverage of the debate, will begin.

VIDEO: Donald Trump questions Ted Cruz's faith