Could America’s longest-serving independent congressman really become the country’s next president?

Six weeks ago, before Bernie Sanders became a Democratic candidate and raised an impressive $15 million, the question would have represented more of a rhetorical scenario than a tangible threat to former secretary of State and presumed presidential nominee Hillary Clinton.

But as political analysts trace the campaign trail of the Vermont senator, the trend emerging is as unavoidable and vibrant as the tens of thousands greeting him: Senator Sanders is gaining traction, and the privilege of being the "presumptive nominee" has become increasingly up for grabs.

Nearly 10,000 people filled an arena in Madison, Wis. last Wednesday. Then, more than 2,500 in Council Bluffs, Iowa, and this week, another 7,500 in Portland, Maine.

“Mr. Sanders is now doing nearly as well as Barack Obama did among liberal voters in 2008. That makes him competitive in relatively liberal contests, like the Iowa caucuses or the New Hampshire primary,” wrote The New York Times reporter Nate Cohn.

Even Mrs. Clinton has acknowledged Sanders’ potential. “This is going to be competitive. It should be competitive,” she told MSNBC in Iowa on Tuesday.

“We are worried about him, sure,” Clinton’s Communications Director Jennifer Palmieri told MSNBC’s Morning Joe on Monday. “He will be a serious force for the campaign, and I don't think that will diminish.”

Now, all eyes are turning to the senator’s still-nascent campaign infrastructure, the importance of which Clinton stressed in a CNN interview this week and a point that even Sanders admitted could be overwhelmed.

“We've only been a declared candidate for two months,” Sanders told The Associated Press on a campaign stop in Iowa. “In a certain sense, our momentum is outpacing our infrastructure.”

Currently, Sanders’ team appears as a squint compared to that of the former secretary of state, but more staffers are scheduled to come aboard next month, said his campaign manager Jeff Weaver.

The senator has slightly more than 50 paid staffers overall, while Clinton has nearly 50 organizers in Iowa alone, as well as at least one in every other state.

Whether Sanders will be able to convert his arena audiences into active voters remains to be seen, but it will be critical to his success, analysts say. In terms of national poll numbers, the senator still lags behind Clinton by an average of more than 40 points, according to NPR News.

But Sanders is also rising among voters in New Hampshire, his neighboring state, where his gap with Clinton is now only eight points, reported NPR. In Iowa, writes Cohn, his support over the past month has doubled.

Soon, however, pundits predict Sanders will hit a brick wall – one standing within the Democratic establishment and etched with Clinton’s highly recognized initials.

Sanders’ messages about equality for the middle class and economic disparity have been resonating particularly well with Millennial voters, who describe him as “what this generation needs” and gleefully chant the slogan: “feel the Bern.”

But many of the older and more conservative Democrats aren’t taking him seriously. William Arnone, a voter interviewed at Clinton’s campaign kickoff rally in New York, told NPR News that his 21-year-old daughter “broke my heart the other day.”

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“She said, ‘Daddy, I met Hillary, I love her, I know you do too. I like Bernie Sanders,’ ” Mr. Arnone said with a chuckle. “I said, ‘Well, that’s okay, Alison, but let’s get real at some point.’”

This report contains material from The Associated Press.