Former U.S. Secretary of State and Democratic candidate for president Hillary Clinton walks in the Fourth of July Parade in Gorham, New Hampshire, July 4, 2015. Sen. Bernie Sanders' drawing power in Iowa and other early contest states has the Clinton camp worried. Reuters

Friends and advisers of former Secretary of State Hillary Clinton are getting anxious about her closest rival in the Democratic presidential primary contest, Sen. Bernie Sanders. Once thought to be a long shot with little chance of giving Clinton much competition, Sanders is pulling in huge crowds in early states, creeping up in the polls and giving Hillary supporters something to think about, according to the New York Times.

In contrast to the booming crowds at Sanders' campaign events, Clinton’s strategy so far has been more focused on having small meet-and-greets with voters where she listens to their concerns in an attempt to evoke humility and an appreciation for the everyday Americans' concerns.

I think we underestimated that Sanders would quickly attract so many Democrats in Iowa who weren’t likely to support Hillary,” an anonymous Clinton adviser told the Times. “It’s too early to change strategy because no one knows if Sanders will be able to hold on to these voters in the months ahead. We’re working hard to win them over, but yeah, it’s a real competition there.”

The Iowa caucuses are the first official test of candidate strength in the presidential primary season, and winning there can be a boost to a candidate's momentum. In 2008, Clinton notably lost there to then-Sen. Barack Obama. With the rise of the Sanders campaign, Clinton has had to face the dilemma of whether or not to strike back at Sanders, potentially alienating his liberal base and granting a sense of legitimacy to her opponent.

The Sanders momentum isn’t confined to Iowa. Last week he packed 10,000 people into a Madison, Wisconsin, arena. In Minnesota, he entertained 5,000. Sanders Friday hit his own personal Iowa record with 2,500 people in a Council Bluffs convention center.

In an interview on MSNBC’s “Morning Joe,” Clinton campaign director Jennifer Palmieri said they had expected the challenge all along. “Of course we're worried about him. This is an election. He is doing well. And we'll have to, you know, we'll have to make our case,” she said.

Sanders has been catching up to Clinton since his announcement in May. The month before he joined the race, Sanders polled nationally at 8 percent. That nearly doubled the next month.

In Iowa, Sanders has made up ground from the 6 percent at the beginning of May to 20 percent now. Clinton has dropped from nearly 61 percent down to just about 54 percent, a Real Clear Politics average of polls indicates.

Polls put him just above 11 percent at the beginning of May in New Hampshire while Clinton pulled in just over 51 percent. A current Real Clear Politics average of polls in New Hampshire indicates Sanders has closed the gap significantly, with 30.5 percent to Clinton’s 46 percent.