By Evan Halper and Chris Megerian, Tribune Washington Bureau (TNS)

The firewall that Hillary Clinton spent months painstakingly constructing to ensure quick, early and decisive victory in the Democratic nominating contest isn’t holding, leaving the candidate once considered the prohibitive favorite scrambling to regain her momentum.

Just weeks before ballots are cast in the key early-voting states of Iowa and New Hampshire, Clinton faces risk of defeat in both places, where anything but convincing victories for her could herald the kind of drawn-out, bloodying primary that establishment Democrats had banked on averting.

Clinton’s change of fortune comes even as she has run a disciplined campaign that dominates in fundraising and reaching voters with expertly produced advertising. But voters are unenthusiastic. She has not touched off the kind of excitement generated by Vermont Sen. Bernie Sanders, who is successfully engaging new supporters the way Barack Obama did when he bested then-front-runner Clinton in Iowa in 2008.

“It’s like a quarter flipping around in the air,” George Ensley, chairman of the Democratic Party in Boone County, Iowa, said of the race in that state, where a comfortable lead Clinton enjoyed since spring has evaporated. “Which way is it going to come down? I don’t know right now.”

Sanders, the reconstructed New Dealer, has the Clinton juggernaut in a heightened state of concern. He is leading in New Hampshire in most every independent poll, and polling in Iowa suggests the race there is a toss-up, with the momentum on his side. Nationwide, Clinton’s lead over Sanders has slipped considerably.

The slipping poll numbers have pushed Clinton to take a feistier — and riskier — approach to confronting rival Sanders, a democratic socialist once looked upon by Clinton operatives as a model adversary: too far outside the mainstream to pose a significant threat, but popular enough to give the appearance of a real race.

“As we have gained momentum, I think it’s fair to say the Clinton campaign has become very nervous,” Sanders told NBC on Tuesday night. Clinton denies the developments are causing any unexpected heartburn, saying polls go up and they go down, and repeating her assertion that the campaign always anticipated a tight race.

Yet her approach has undeniably changed.

She is now relentlessly attacking the rival she mostly ignored for months. By this week, even Chelsea Clinton had touched off a spirited back-and-forth with the Sanders campaign. Speaking to voters in New Hampshire, she accused her mother’s rival, who favors a European-style single-payer health care system, of seeking to “dismantle Obamacare … dismantle Medicare and dismantle private insurance.”

The comments riled Sanders, as well as his network of activists who accuse the Clinton machine of misleading voters.

Clinton herself is seizing on Sanders’ mixed record on gun safety, mocking his explanation that he has not always voted for tighter restrictions because he represents a rural state where hunting is part of the culture.

“Sen. Sanders has been a pretty reliable vote for the gun lobby,” Clinton told NBC on Wednesday.

The Clinton campaign also told reporters that Sanders risked not being backed by President Obama, who threatened to withhold support from any candidate who doesn’t agree with his views on gun control — an interpretation the White House did not exactly stand behind.

Clinton also accuses Sanders of reckless economics. “There’s no way, if you do the arithmetic, how to pay for what he has proposed without raising taxes on the middle class,” she said at a campaign event Monday in Waterloo, Iowa.

The Sanders campaign is responding aggressively. Amid all the demands from Clinton for an apology for past gun votes by Sanders, the Vermonter proudly waves his D-minus rating from the National Rifle Association at campaign events and has embraced a robust gun-control agenda.

He also demanded Clinton make an apology of her own. He pasted on his Twitter account a mailer from 2008 in which Clinton positioned herself as a champion of gun rights and suggested she explain herself.

The evolution of the race into a political street fight carries risks for both candidates.

The initiation of the barbs by Clinton has the taint of desperation, coming from a candidate who did everything by the book yet is watching her lead slip away nonetheless. It is unclear whether they will be effective in helping stir up the voter passion her campaign has lacked.

“This is a real challenge for Hillary Clinton,” said Democratic pollster Peter Hart. “I don’t think you can turn Bernie Sanders into a villain. He isn’t one. When you are 74, as he is, and you are a socialist from Vermont, it is hard to make you out to be Richard Daley or some other political boss,” he added, referring to the former Chicago mayor famed for his decadeslong stranglehold on the city’s political machine.

As for Sanders, he appears to be going back on his repeated vow not to engage in negative campaign tactics, and he is clearly less comfortable in such brawls than Clinton is.

But it is also difficult to gauge whether the Sanders surge will translate into votes, particularly in the caucus state of Iowa, where the mere act of voting is daunting and time-consuming — and Clinton’s get-out-the-vote operation is more seasoned and better-funded.

“Are (his) people going to show up to a caucus?” said Tom Henderson, Democratic Party chairman in Polk County, which includes Des Moines. “Or are they people who like to show up at rallies and cheer them on, and then leave?”

©2016 Tribune Co. Distributed by Tribune Content Agency, LLC.

Photo: U.S. Democratic presidential candidate Hillary Clinton addresses supporters at the Electric Park Ballroom in Waterloo, Iowa January 11, 2016. REUTERS/Aaron P. Bernstein