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 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.

“There is nobody better than Bernie at delivering a message,” said Richard Tarrant, the businessman 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 hopes 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 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 kleig 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 favour.

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 recalled a speech Sanders had made on the House floor on Oct. 9, 2002, laying out the reasons that he opposed the American 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 Obama, who opposed it.

Sanders has put a bigger dent in Clinton’s armour of inevitability than most expected when he first 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, whom she served as secretary of state.

Clinton also has struggled with a controversy over her use of a private email 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.