Clinton turns up the heat as her lead has evaporated

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.

She is now relentlessly attacking the rival she mostly ignored for months and the Sanders campaign is responding aggressively.

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. It is unclear whether they will be effective in helping stir up the voter passion her campaign has lacked.

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.