Washington (CNN) Vermont Sen. Bernie Sanders' bid to topple Hillary Clinton's front-runner status is hitting the marks of a serious contender: He's closed the fundraising gap. He's competitive and in some cases leading in early state polls. He's still drawing crowds of thousands to campaign rallies.

And Tuesday night in Las Vegas, when Democrats meet at the CNN-hosted debate, he'll face his biggest test yet: going toe-to-toe with the former secretary of state.

But don't expect fireworks, Sanders said Wednesday -- it's not his style.

"You're looking at a candidate who has run in many, many elections, who has never run a negative political ad in my life and hopes never to have to run them," Sanders said when CNN asked about his debate tactics. "And you're looking at a candidate who does not go about attacking people personally. I just don't do that."

He said he'll make one big point: He's held liberal positions on economic issues and civil rights for decades.

The first one he brought up: The Glass-Steagall Act, which separated commercial and investment banking until it was repealed under President Bill Clinton. He wants to re-institute it, but his rival Hillary Clinton has said she wouldn't. Progressives view re-instating that Depression-era law as a key part of reforming Wall Street.

Sanders said he'll highlight "which candidate has the track record and the history and the ideas of standing up to those very powerful special interests and fighting for an economy that works for all Americans rather than just the top 1%. And I think in that regard, Secretary Clinton and I have differences in terms of Wall Street."

The best evidence of Sanders' potency comes in polls out of Iowa -- where he is within single digits of Clinton -- and New Hampshire, where he's led since August. His seeming viability is causing some Clinton allies to wonder whether she'd be best paring back her presence and conceding the Granite State's first-in-the-nation primary so she could focus her efforts elsewhere.

Both Iowa and New Hampshire's electorates tend to be whiter and more liberal than the country overall, and those demographics play to Sanders' strengths.

But after the first two contests, Clinton's advantages come into play: Her better-funded super PAC can make expensive ad buys, helping boost her in multiple states at a time. Her strong support among African-Americans becomes a factor. Her endorsements from elected officials and party leaders can carry more weight in states where candidates have less time to campaign. Her campaign infrastructure -- Clinton's campaign blanketed the country with temporary field organizers when it launched -- might begin to pay off.

So Sanders is working to expand his base.

On Wednesday, he spoke at the Congressional Hispanic Caucus Institute's annual conference in Washington, courting an audience of hundreds of Latino voters in a 44-minute speech.

Sanders attacked Republican presidential front-runner Donald Trump during the speech and afterward, calling his comments about undocumented immigrants "racist."

And he played up his support for an overhaul of U.S. immigration laws.

"I think the Latino community is obviously deeply concerned about comprehensive immigration reform and a path toward citizenship is something I strongly support," Sanders said. "And I think the Latino community is outraged over some of the racist attacks that have been made against some Latino groups -- and that is unacceptable."

The Vermont senator won at least a few converts.

Karina Preciado-Quintanilla, an academic adviser at Brandman University in California and a master's student herself, asked Sanders in the rope line how she'd be able to escape the burden of student loan debt and afford a house.

"He put his hands on my shoulders and he asked what my current interest rate was," she said. "I said it was fixed at 6.8%. He said that the new reform would be to un-fix it and get the lowest rates possible."

More than anything, she said, she loves his style.

"That is the whole appeal. He is willing to hold up that mirror," she said. "He still has that fire, that passion for real reform."

Liliana Montiel, a 22-year-old student at Wichita State in Kansas, said she had long supported Clinton.

"But Bernie's policies resonate with me more. And, man, he's got the guts," she said.

"He's going after corporate America, which I haven't seen in my lifetime, in my generation," Montiel said. "He's definitely won me over, 100%."