DES MOINES — A mere 240 people live in the rural northeast Iowa town of Kensett, so when more than 300 crowded into the community center to hear Senator Bernie Sanders of Vermont, many driving 50 miles, the cellphones of Democratic leaders statewide began to buzz.
Kurt Meyer, the county party chairman who organized the Saturday event, texted Troy Price, the Iowa political director for Hillary Rodham Clinton. Price called back immediately.
“Objects in your rearview mirror are closer than they appear,” Meyer said he had told Price about Sanders. “Mrs. Clinton had better get out here.”
The first evidence that Clinton could face a credible challenge in the Iowa presidential caucuses appeared late last week in the form of overflow crowds at Sanders’s first swing through that state since declaring his candidacy for the Democratic nomination.
He drew 700 people to an event Thursday night in Davenport, for instance — the largest rally in the state for any single candidate this campaign season, and far more than the 50 people who attended a rally there Saturday with former governor Martin O’Malley of Maryland.
The first-in-the-nation caucuses, on Feb. 1, loom as a major test for Clinton: She came in third in Iowa during her presidential run in 2008, and anything less than a decisive victory this time would rattle her shell of inevitability and raise questions about her strengths as a standard-bearer for an increasingly liberal Democratic Party.
Ten declared and likely candidates from both parties appeared on television news programs on Sunday to talk about a range of issues such as how to fight the Islamic State and what to do about the immigration problem.
“We need a lot more debates in this campaign,’’ Sanders said on NBC’s ‘‘Meet the Press.’’ He said the Democratic debates should begin as soon as July and some Republicans should be in the mix. So far, the Democratic Party has said it will hold six presidential primary debates beginning in the fall. The first Republican debate is scheduled to be in August.
Sanders also said in response to a question that his 1972 essay dealing with rape fantasies was “poorly written,” but was meant to discuss dominance and gender stereotypes. Published in an alternative newspaper called the Vermont Freeman, the essay caused a stir last week when Mother Jones unearthed it.
O’Malley, who entered the Democratic contest Saturday, said on ABC’s “This Week’’ that he has achieved more than Sanders. ‘‘I have a track record of actually getting things done, not just talking about things,’’ O’Malley said.
Sanders is considered the Senate’s most left-wing member, and he has been inspiring fervor among the Democratic base at recent rallies and town hall-style meetings, including Wednesday in the first presidential primary state, New Hampshire.
Clinton is far ahead in the polls, fund-raising, and name recognition, however, and she is expected to continue to have a much more organized and sophisticated campaign operation in Iowa and nationwide than Sanders does.
Her mix of centrist and liberal Democratic views may yet prove more appealing to the broadest number of party voters as well, while some of Sanders’s policy prescriptions — including far higher taxes on the wealthy and deep military spending cuts — may eventually convince Democrats that he is unelectable in a general election.
Even before Sanders drew unexpected levels of support at his Iowa events, advisers to Clinton’s presidential campaign were emphasizing that they expected the caucuses to be competitive.
“We’ve said from day one that we take nothing for granted, and two, we’re humble, which is a direction from Hillary herself,” said Matt Paul, Clinton’s Iowa director.
Judging from Sanders’s trip to Iowa last week, there is real support for his message — though some Democrats also simply want to send a warning shot to Clinton to get her to visit here more.
Clinton’s large Iowa staff, which arranged her earlier visits to the state when she met with small groups on a “listening tour” in carefully controlled settings, has taken the posture of not overreacting to Sanders or taking him too seriously.
The large crowds for Sanders were a sign of many voters’ desire to hear and meet Democratic candidates in free-flowing town hall-style gatherings, with policy issues discussed in detail, which Clinton has avoided. Her campaign has promised that such events will follow this summer.
Sanders’s stop at a brewery in Ames on Saturday was so mobbed that more than 100 people who could not fit inside peered through the windows.
Clinton’s advisers are most concerned that Sanders might prove to be effective at painting Clinton as squishy or untrustworthy on liberal issues.
The crowds at Sanders’s Iowa events appeared different from the state’s famously finicky tire-kickers. Many said they had already made up their mind to support Sanders.
They applauded his calls for higher taxes on the rich to pay for 13 million public works jobs, for decisive action on climate change, and for free tuition at public colleges.