(CNN) Joe Biden and Elizabeth Warren have been circling for months -- like battleships just out of range.

We now know that will change in the second week of September, when the Democratic front-runner and the rival who is stirring the loudest buzz on the campaign trail meet onstage, at last, in the next Democratic debate.

For the first time, Democratic voters will see side by side two candidates who perhaps best personify many of the ideological, gender, thematic and stylistic contrasts in their nominating race.

Debate host ABC News is expected to confirm Thursday that only 10 candidates made the cut after a qualifying period that set fundraising tests a nd minimum polling thresholds, resulting in a single debate night for the first time in the nominating process.

In the previous two rounds of double-header debates on consecutive nights, Biden and Warren were drawn apart, leaving the most intriguing emerging dynamic in the race unaddressed.

Former Vice President Biden, 76, is the epitome of the Democratic establishment, a moderate, portraying his pragmatism and appeals to America's "better angels" as the key to wooing President Donald Trump's voters and beating a vulnerable incumbent in 2020.

Warren, 70, a crusading reformer of the financial system, is in the same generation as Biden, who first ran for president in the 1980s, but campaigns with the relentless energy of a fresher figure on the national stage.

Biden is the quintessential old-school, sentimental back-slapping politician, a former Senate bull with a trove of stories from Washington's corridors of power who is pledging to restore the civility of a pre-Trump era.

Warren, a senator from Massachusetts, wants to shoulder hopes of shattering the highest glass ceiling in politics from Hillary Clinton, has a plan for every problem and is driving the liberal tide sweeping her party.

The former vice president's campaign is polishing his already storied reputation as a gaffe machine. After detonating a new rhetorical landmine as he forgot just where he had spoken at Dartmouth, he told a crowd in New Hampshire: "I want to be clear: I'm not going nuts."

As a former college professor, Warren, by contrast, is a stickler for detail. Her blitz of policy papers spawned her line "I've got a plan for that," which has become an unofficial motto of her campaign.

While popular among Democrats, partly through serving as Barack Obama's vice president, Biden has yet to demonstrate electric appeal among liberal base voters but may be able to call on a silent army of more moderate Democrats less active in the primary race.

Warren, however, is starting to attract big crowds -- including an eye-catching 15,000-strong turnout at a town hall in liberal Seattle recently. Her candidacy is a bet that Americans are ready to reject Biden-style centrism for a more radical platform on issues from student loans to climate change.

Health care lays bare Biden and Warren divides

Most recent national and early-voting-state polls of the Democratic race have Biden a clear front-runner and Warren consolidating her steady rise, disputing second place with Bernie Sanders.

In a Quinnipiac University survey published Wednesday , Biden led with 32%, ahead of Warren with 19% and Sanders with 15%.

The Biden/Warren rivalry has been building quietly for months -- with differences emerging especially over health care.

Biden is warning against "Medicare for All" options backed by Warren that could threaten the private health insurance plans of millions of Americans, and he wants to expand Obamacare and add a public Medicare option.

"You worked like hell. You gave up wages for it," Biden told union members in Iowa last week, pledging they could keep their union-negotiated health insurance.

Warren is warning that Democrats should not spread scare stories about people losing their insurance.

"We should stop using Republican talking points in order to talk with each other about how to best provide that health care," Warren said at the CNN Democratic debate last month.

Pre-debate buildup will highlight such differences to talk up the prospect of clashes between Biden and Warren in what could be a defining early moment of a race that is beginning to shed outsider candidates and will intensify after Labor Day.

And it's possible that the candidates many analysts believe could be the last pair standing next year will decide that the time is right to throw off the gloves and fully engage.

Fireworks may have to wait

But given the early stage of the race and potentially different goals of Warren and Biden -- not to mention the other eight candidates bristling for a fight, such fireworks may not erupt.

"I understand the inclination for that to be the hype, but I think it maybe could be a letdown -- it is so tough with 10 candidates on the stage for spotlight to be on those two candidates," said Aaron Kall, director of debate at the University of Michigan, who writes books about presidential debates.

The former vice president played down expectations of a one-on-one duel with Warren in Houston.

"I'm just going to be me and she'll be her and let people make their judgments. I have great respect for her," he told reporters in South Carolina on Wednesday.

But Biden did appear to preview a possible policy duel with Warren later in the race.

"I'm looking forward to getting to the place, assuming I'm still around, that it gets down to a smaller number of people so we can have more of a discussion instead of one-minute assertions," he said.

One reason why the Biden vs. Warren showdown may have to wait is that Warren has yet to fully establish herself as the progressive champion in the Democratic race. Sooner or later as first votes approach next year, she will have to dispatch the challenge from Sanders i f she is to move on, and her first job next month may be to appeal to supporters of the senator from Vermont.

Biden has already learned that front-runners can expect attacks from all sides and will have more to worry about than just Warren.

His campaign suffered an early wobble when Sen . Kamala Harris of California jabbed him on race in an earlier debate, though she has been unable to build on her resulting momentum.

Biden will be keen at the September debate to step up his battle against Trump, to cement perceptions among many Democrats that he is the candidate best equipped for the general election.

In an interview with black journalists on Tuesday, Biden argued he was the "most electable" Democrat -- and in a veiled possible swipe at Warren said his rivals might get bigger crowds or excite progressives but he has the most diverse coalition.

"I have never, ever, ever in my entire life had a circumstance where I have felt uncomfortable in the black community," Biden said. Both Warren and Sanders poll poorly among African American voters.

Warren is trying to counter Biden's narrative that he is the strongest potential opponent for the President, declaring last week that "it's not enough to be 'not Trump.' "