Former New York City Mayor Michael Bloomberg knows his way around a debate stage. In his three previous bids for office, he faced off against competitors more than half a dozen times, both as a Republican and then later an Independent. His former rivals say that while Bloomberg might not shine on the debate stage, he certainly comes prepared.

Running for president as a Democrat, Bloomberg will suit up for the primary debate on Wednesday night—his first time facing five other candidates—thanks to a tweak last month to the national party’s debate qualification rules.

While Bloomberg officially qualified for the debate on Tuesday morning, when the latest NPR/PBS Newshour/Marist poll showed him with 19% support nationally, second only to Vermont Sen. Bernie Sanders, the NY Times reports that his campaign has been steadily prepping him to take the stage, running mock debates with stand-in candidates.

While his style is crisp and sometimes curt, those who have debated him before said the billionaire businessman’s minimalism should not be underestimated.

Take 2001, when he faced then Public Advocate Mark Green in a general election debate. For months, Green held a yawning lead over Bloomberg in polls, boosted by the strength of the Democratic party and name recognition from decades of work in the public sector. But after a bruising primary fight, and in the wake of the September 11th attacks, Green said he took the debate stage with Bloomberg ill-prepared for the fight.

“I didn't do much pre-debate prep on Bloomberg before we debated because he had no serious public record to go after,” Green said. As soon as it began, Bloomberg took a tactical approach to dissecting his opponent, with his first strike aimed at the public advocate’s years of experience in government—the very credentials Green said made him more qualified in his bid for City Hall.

“My opponent has not had any experience in managing a large organization in leading a large number of people, in setting large budgets, in actually doing things,” Bloomberg declared, before he pivoted to his own resume, talking about his successful financial news and information company.

From there, the first-time candidate showed he was a student on the issues, holding his own with Green in questions about topics ranging from education policy to how the city should rebuild lower Manhattan. At a certain point, Green remembers realizing that Bloomberg was outperforming expectations, including his own.

“Because there was a low bar, he excelled," Green recalled. "Because he was crisp, calm, had answers to the most obvious questions and wasn't a deer in headlights.”

Bloomberg is sure to receive criticism from his presidential primary rivals over how he uses his money to campaign. It comes up every time. In a debate against former Bronx Borough President Fernando Ferrer in 2005, when the mayor was again spending historic amounts in the race, Bloomberg was unapologetic about it.

“I'm trying to get my message out to every community in the city. It costs a lot of money. And I don't have a big Democratic machine behind me,” he said.

When reached by phone Tuesday, Ferrer said it didn't matter how Bloomberg performed on the stage Wednesday night in Las Vegas, “he'll just spend more money.”

Former City Comptroller Bill Thompson, who was the last to debate Bloomberg in 2009, told Gothamist/WNYC, “no comment” when reached by phone. But he later appeared on NY1’s Inside City Hall as a “Wise Guy” where his debate experience against Bloomberg was the main topic of conversation.

“I’m glad I spent the last couple of days not talking to the press so I could wind up here tonight,” Thompson told host Errol Louis. He added, “When you go back and look at it, Mike was a formidable debater.”

In 2009, Thompon’s chief criticism of Bloomberg was that the mayor was overruling the will of the people by seeking a third four-year term. Thompson’s tagline in that debate was, “eight is enough,” referring to the two terms the mayor had already served in office.

In an attempt to respond with rhetorical flourish, Bloomberg seized on that line arguing, “Eight isn't enough for better schools, eight isn't enough for lower crime, eight isn't enough for good government in this city, quite the contrary, eight isn’t enough for all the good things that will happen and I think that’s what voters will decide on November 3rd.”

One line of attack Thompson avoided was any reference to the mayor’s history of inappropriate comments about women. On NY1 Tuesday night, Thompson said he’d seen others try those lines of attack against Bloomberg in previous campaigns and fail to leave any mark, “If anything, the public ignored it. It didn’t make sense to go there.” He added that he expected others might try that approach during the presidential debate on Wednesday, but that he didn’t expect it would work for them either.

“After Donald Trump, there is nothing that surprises or stuns people any longer,” Thompson said.

Green, who is backing Senator Elizabeth Warren for president, offered his own assessment of how Bloomberg will approach the debate stage. He expects the mayor will be conversant on the issues and ready for an onslaught of criticism. But he also said Bloomberg will need to keep his notorious short temper in check.

“If he shows any of that upper class Richie Rich annoyance, it really is going to hurt him,” Green said.