"Hillary Clinton has won the Iowa Caucus," the Clinton campaign said. "After thorough reporting -- and analysis -- of results, there is no uncertainty and Secretary Clinton has clearly won the most national and state delegates."
The state party indicated in a separate statement that it was not ready to make a call.
"The results tonight are the closest in Iowa Democratic caucus history," Iowa party chairman Andy McGuire said. "We will report that final precinct when we have confirmed those results with the chair."
One thing is clear after Monday night's Iowa caucuses: there's a long, volatile election season ahead before two deeply fractured parties can unite behind a nominee.
Republican Ted Cruz
bested Donald Trump
, raising questions about the billionaire's reliance on his celebrity instead of traditional political organization. And Marco Rubio
's stronger-than-expected showing could mark him as the establishment's best hope against a grassroots revolt in next week's New Hampshire primary and beyond.
Cruz's victory sets him up as a formidable force in delegate-rich, Southern states to come and offers movement conservatives hope that one of their own can become the Republican nominee for the first time since Ronald Reagan.
Claiming victory, Cruz fired immediate shots at both Trump and the party elites he has so infuriated by waging an anti-establishment crusade that has nevertheless endeared him to the GOP's rank and file.
"Iowa has sent notice that the Republican nominee and the next President of the United States will not be chosen by the media, will not be chosen by the Washington establishment," Cruz said.
With about 99% of the GOP vote in, Cruz was ahead of Trump 28% to 24%. Rubio was at 23%.
"It is breathtaking to see what happens when so many Americans stand up and decide they're fed up with what happens in Washington and they want something different. They want a leader they can trust, they want a leader that stands for them against the corruption of Washington," Cruz told CNN's Dana Bash in an interview aired Tuesday on "New Day."
Trump, hours after predicting a "tremendous" victory, delivered a short but gracious speech that lacked his normal bombast, saying he loved Iowa and vowed to bounce back next week in New Hampshire.
"We will go on to get the Republican nomination and we will go on to easily beat Hillary or Bernie," Trump told supporters. "We finished second, and I have to say I am just honored."
Rubio will also leave Iowa with a leg up over other establishment rivals including former Florida Gov. Jeb Bush, New Jersey Gov. Chris Christie and Ohio Gov. John Kasich, who have a lot at stake in New Hampshire.
"This is the moment they said would never happen. For months, they told us we had no chance," a jubilant Rubio said. "They told me that I needed to wait my turn, that I needed to wait in line. But tonight here in Iowa, the people of this great state have sent a very clear message — after seven years of Barack Obama, we are not waiting any longer to take our country back."
On the Democratic side, Clinton and Sanders are deadlocked at 50% with 99% of the votes counted. Clinton, the national front-runner, admitted breathing a "big sigh of relief" after escaping Iowa -- the state she handily lost to Obama in 2008 -- but promised a vigorous campaign with Sanders.
"It's rare that we have the opportunity we do now," she said in a speech that didn't explicitly claim victory but sought to position her as the authentic progressive in the race.
Sanders, who trailed Clinton in Iowa by 30 points three months ago, told a raucous crowd chanting "Bernie, Bernie" that his campaign made stunning progress.
"Nine months ago, we came to this beautiful state, we had no political organization, we had no money, we had no name recognition and we were taking on the most powerful political organization in the United States of America."
"And tonight," he said, "while the results are still not known, it looks like we are in a virtual tie."
Though Sanders fared well in Iowa and is nicely posited in New Hampshire, his hurdle is proving that he can appeal to more ethnically diverse electorates in later contests in places such as South Carolina.
Sanders made the case to CNN's Chris Cuomo, when he campaign plane landed in New Hampshire early on Tuesday morning, that he expects to challenge Clinton among nonwhite voters.
"We lost (the nonwhite vote), but that gap is growing slimmer and slimmer between the secretary and myself. I think you'll find as we get to South Carolina and other states, that when the African-American community, the Latino community, looks at our record, looks at our agenda, we're going to get more and more support," Sanders told Cuomo on "New Day."
The caucuses resulted in two casualties -- one on each side. Former Maryland Gov. Martin O'Malley, a Democrat, and former Arkansas Gov. Mike Huckabee, a Republican, both dropped their candidacies after faring poorly.
Even before the caucuses began, Ben Carson's campaign said he wouldn't go directly to New Hampshire or South Carolina -- the site of the next primary contests. Instead, the retired neurosurgeon, who was briefly the Iowa front-runner last fall, will go to Florida to rest and see his family.
Former Pennsylvania Sen. Rick Santorum is also skipping New Hampshire but will go straight to South Carolina, which holds its Republican presidential primary on February 20.