Sen. Bernie Sanders (I-Vt.) speaks on Tuesday, May 26, 2015, in Burlington, Vt., where he formally announced he will seek the Democratic nomination for president. (AP Photo/Andy Duback)

After Hillary Clinton kicks her campaign into high gear Saturday with an official launch rally in New York City, she will travel to Iowa and run up against a challenger who has become an unexpected lightning rod on the left: presidential long-shot and self-described socialist Sen. Bernie Sanders (I-Vt.).

"I freely admit what everybody here knows: that Secretary Clinton goes into this campaign as the heavy favorite. The polls today show her way out in front,” Sanders told reporters Thursday during a breakfast hosted by the Christian Science Monitor. "But I will tell you: If you look at these very same polls, we have momentum. And our numbers are growing."

In the six weeks since Sanders announced his presidential campaign, the Vermont independent has emerged as the leading and unlikely rival for the hearts of progressives in the Democratic Party — bolstered by angst on the left and concerns over Clinton's populist bona fides. Enthusiastic supporters have created a special Twitter hashtag -- #FeelTheBern -- to push his candidacy.

In part, Sanders's rise fills a void left by Sen. Elizabeth Warren (D-Mass.), whose decision not to make a White House run left her supporters yearning for an alternative to the former secretary of state, whom they see as too moderate.

“We are drawing very large crowds. I am surprised by the size of the crowds … which as I understand it, I may be wrong, are the largest crowds of any candidate,” Sanders said, sounding incredulous at the thought. “If I were here six weeks ago and we were discussing my presidential ambitions, I would not have known whether I was running for office or not.”

Attendance at Sanders's free-wheeling rallies in early voting states have reached into the thousands. The contrast is striking: So far, Clinton has only held small "roundtables" focused on policy issues, all carefully coordinated by her team in advance.

That dynamic may change after Clinton's highly publicized launch, which brings an end to her so-called "ramp-up" period. She will follow that rally with a two-day swing through Iowa and several more days of campaigning in New Hampshire, South Carolina and Nevada. Sanders, for his part, is slated to speak at a half-dozen town halls this weekend alone.

Meanwhile, the Sanders campaign is beginning the process of hiring on-the-ground staffers in Iowa and New Hampshire.

[Bernie Sanders: ‘Now is not the time for thinking small.’]

Most recently, he and his aides were also heartened -- and surprised -- by an unexpectedly strong showing in a straw poll at Wisconsin's Democratic convention last week, where he lost to Clinton among delegates by just eight points, 49 percent to 41 percent. ("Imagine if we had actually worked that?" said Sanders spokesman Michael Briggs when asked about the contest.)

The numbers are still stacked against him -- almost insurmountably so. A national Quinnipiac poll released several weeks ago showed Clinton completely dominating the Democratic field, taking 57 percent support among Democratic voters. Sanders, a self-described democratic-socialist, follows her in second place with 15 percent support.

But he has seen a seven-point jump in that poll since April, before he had entered the race. That puts him in front of Vice President Biden and former Maryland governor Martin O’Malley, who has also positioned himself to Clinton's left but has thus far failed to capture the attention of many Democratic voters.

The same survey suggests that 62 percent of voters have not heard enough about him to make up their minds. (The national survey of 748 Democratic voters was conducted from May 19 to 26 and has a margin of error of plus or minus 3.8 percentage points.)

[Hillary Clinton only beat Bernie Sanders by 8 points in a straw poll. So?.]

"I would also remind you that polls have a lot to do with name recognition. I think the secretary may well be one of the best-known people in the United States of America," Sanders said. "If you do a poll in various states, 50, 60, 70 percent of people don't even know who I am."

That's not to mention the difficulty Sanders will likely face funding his campaign. An avid critic of super PACs, Sanders has committed to forgoing big-dollar donors in favor of grass-roots fundraising.

Clinton has, for her part, remained keen to stress her efforts to win over voters in the early states, and her campaign has signaled that it is ready for a competitive challenge. In anticipation of Clinton's address on Saturday, her team has been boasting about a nationwide organization that includes 15 offices in early-voting states, with 50 organizers and more than 10,000 committed volunteers. Following her rally in New York, Clinton will attend a house party in Sioux City. On Sunday her campaign will host a "launch party" at the Iowa State Fairgrounds in Des Moines before Clinton attends another house party in Burlington.

Sanders's Hawkeye State visit will begin in Des Moines on Friday evening; on Saturday he will visit Marshalltown and Cedar Rapids, and on Sunday he will speak in Waterloo, Iowa Falls and Indianola.

Despite the odds, the Vermont senator insists he's serious.

"I respect people on the right, people on the left who want to use a campaign as an opportunity to get their ideas out. There’s nothing wrong with that," Sanders said.

But referring to his own bid, he said: "This is not an educational campaign. This is not a protest. This is a campaign to win. I do believe we can win this election."