Democratic presidential candidate Sen. Bernie Sanders, I-Vt. speaks during a rally, Friday, Oct. 9, 2015, in Tucson, Ariz. (Rick Scuteri/AP)

Vermont Sen. Bernie Sanders (I) has been one of the biggest surprises of the early presidential campaign season, as he steadily rises in the Democratic primary polls and draws adoring throngs numbering tens of thousands to his rallies across the country.

On Tuesday, with the lead-off of six scheduled Democratic debates, voters will have their first chance to take the measure of the passionate newcomer to the national scene alongside front-runner Hillary Rodham Clinton.

For the former secretary of state, the debate stage is a familiar setting and one where she has performed well in her previous races for senator and for president. But it is also well-suited to Sanders, say his aides and those who have seen him square off against opponents in Vermont.

“He speaks from a certain kind of certainty, a declarative voice, steeped in a certain amount of outrage,” said Greg Guma, a liberal activist and journalist in Vermont whom then-Gov. Madeleine Kunin recruited to play Sanders in debate rehearsals when the mayor of Burlington challenged her as an independent in 1986 (Sanders came in a distant third).

Kunin’s impulse was to dig deep into her policies and trace her rationale, which made her appear less resolute in their practice sessions. “One of the takeaways was: Don’t explain so much. The parallels are there” for Clinton, said Guma, who supports Sanders’s presidential bid.

Supporters are seen at Democratic presidential candidate Sen. Bernie Sanders, I-Vt. rally, Friday, Oct. 9, 2015, in Tucson, Ariz. (Rick Scuteri/AP)

“There is nobody better than Bernie at delivering a message,” said Richard Tarrant, the businessman whom Vermont Republicans nominated to go up against Sanders for an open Senate seat in 2006. “I happen to hate the message.”

Tarrant’s own experience, he said, suggests that Clinton’s best strategy is to get under Sanders’s skin in the hope of provoking an unpresidential outburst.

“Oh, yes. You’ve got to accuse him of something. You point at his face, you accuse him, and he goes nuts,” Tarrant said. Then again, Tarrant lost to Sanders by more than 30 percentage points.

Both Clinton’s and Sanders’s camps are predicting a far more sedate — and substantive — forum than the past two Republican debates. The leading Democratic candidates have studiously avoided directly attacking each other thus far.

That is not likely to change when the klieg lights go on Tuesday night in Las Vegas, said Tad Devine, a veteran political consultant who is working for Sanders.

While the two will not shy from talking about their policy differences, Devine added, “this could be a civil exchange. Bernie’s not headhunting. I don’t see him out there trying to see how many times he can say ‘Iraq war’ ” to remind liberal Democrats that he voted against it while then-senator Clinton came down in favor.

The campaign sent a different signal Saturday when it issued a news release titled “Sanders’ Foreign Policy Experience.” It did not mention Clinton directly, but it recalled a speech that Sanders made on the House floor on Oct. 9, 2002, laying out the reasons that he opposed the U.S. invasion of Iraq.

Clinton’s support of the invasion — a decision she later said she regretted — became a major obstacle in her 2008 quest for the Democratic nomination and a key point of distinction between her and then-senator Barack Obama, who opposed it.

Sanders has put a bigger dent in Clinton’s armor of inevitability than most expected when he announced his candidacy in late May. His populist message and unapologetic embrace of bigger government has brought an outpouring of support from the left.

Meanwhile, though Clinton has shifted to more liberal positions on a number of issues, she remains the face of the Democratic establishment, with its ties to corporate and Wall Street interests. Nor has she succeeded in drawing a clear picture of how her administration would distinguish itself from that of her husband, Bill Clinton, or President Obama, for whom she served as secretary of state.

Clinton also has struggled with a controversy over her use of a private e-mail system, which has stirred public misgivings about her trustworthiness.

The leading contenders will not be the only ones on the stage Tuesday. Clinton advisers say they are preparing for what, in effect, could be two separate debates going on at the same time — one between her and Sanders and the other involving the other three contenders, all of whom are making barely a mark in the polls.

That means that former Maryland governor Martin O’Malley, former Virginia senator Jim Webb and former Rhode Island senator and governor Lincoln Chafee will be looking for a breakout moment — and the best way to do that is to set off some fireworks.

O’Malley in particular seems eager to capi­tal­ize on an opportunity that he described as “make or break” for his struggling campaign.

“Right now, the people in my party, the only two candidates people have heard of are the inevitable front-runner and the senator from Vermont,” he said. “Once the debates happen, people will be able to hear from all of the candidates.”

For Sanders, preparation began in earnest at an Oct. 2 meeting in Burlington with Devine and campaign manager Jeff Weaver, who served as Sanders’s House and Senate chief of staff and has been working on his election efforts going back to 1986.

Devine said the senator requested briefing materials on issues that are likely to arise during the debate and has been having conversations with policy experts. The campaign has declined to name who is offering advice — in part, advisers acknowledge, in respect for the sensitivities of those who do not want to get on the wrong side of the formidable Clinton operation.

Sanders planned to arrive Saturday night in Las Vegas and to buckle down for debate prep sessions Sunday and Monday.

He and his advisers are trying to figure out what questions are likely to come up, and how his opponents will probably respond. There were no plans, however, for the kind of formal rehearsals that Clinton reportedly is doing, with advisers playing the roles of the other candidates.

“He just doesn’t want to do that stuff. It’s just not him,” Devine said. “He knows what he needs.”

Clinton’s skills are well-tested.

“She’s a very good debater. She is firm. She is fluent. She, by and large, did very well in those debates” during the Democratic primary of 2008, said David Axelrod, who was Obama’s chief campaign strategist. “Where she sometimes has difficulties is where she has to make political calculations on the spot on issues that are politically fraught.”

During that earlier race, for example, she appeared unprepared when she was asked whether she backed a proposal by then-New York Gov. Eliot L. Spitzer to grant driver’s licenses to illegal immigrants.

Her equivocal answers drew fire from the other candidates. Clinton now says she favors giving them driver’s licenses.

Sanders and his team believe that one area where he has an advantage over Clinton is the decades-long consistency of his positions: on trade, the Keystone XL pipeline, gay marriage and other areas where she has shifted to the left.

“It’s hard to throw him off his stride, because his stride hasn’t changed,” said Sanders spokesman Michael Briggs.

The most uncomfortable areas for Sanders, Axelrod said, are “places where he’s made concession to politics, because that runs against his brand. But there aren’t many of them,” with the exception of gun control, on which the senator’s moderate position reflects the rural state that he represents.

Axelrod also said Sanders would benefit by adding touches of humor and humanity. “He doesn’t often invoke real people, so there is this sort of theoretical dimension to his rhetoric, and that’s not necessarily helpful at these sorts of events.”