Hillary Clinton's new barrage against Bernie Sanders, the Democratic presidential primary opponent she has all but ignored through most of her campaign, is having an effect - though probably not the one she intended.

Sanders' underdog campaign said it is seeing a surge of contributions as a direct result of the new attention it is getting from the Democratic front-runner, with money coming in at nearly four times the average daily rate reported in the last quarter of 2015.

In its email appeals for money, the campaign accused the Clinton campaign of making "vicious and co-ordinated attacks"on Sanders' health-care plan, which calls for a government-run system. Sanders' strategists are also considering rolling out advertising beyond the early-contest states where it is airing spots now.

Clinton and her team have stepped up their criticism of Sanders on a variety of fronts in recent days as polls have begun to show him edging even with her in Iowa - and, for the first time, looking competitive in a national poll. But the Clinton strategy may be backfiring in some ways.


"Thanks, Team Clinton," said Sanders spokesman Michael Briggs.

"As of now, we are at about US$1.4 million raised since yesterday when the panic attacks by the Clinton campaign began," Briggs said. "We've got 47,000 contributions. We're projecting 60,000 donations. Even for our people-powered campaign, this is pretty darn impressive."

Sanders strategist Tad Devine said the campaign might go on the air with TV ads outside the three early-contest states of Iowa, New Hampshire and Nevada.

Sanders' team now feels pressure to put out its own message across the map before Clinton has a chance to define it on her terms. "That is something we are considering as we speak," Devine said.

A New York Times-CBS News survey released on Wednesday showed Clinton leading Sanders by 48 per cent to 41 per cent among Democratic primary voters. A month ago, that same poll showed her with a 20-point lead nationally.

Overnight (NZT), the authoritative Des Moines Register poll is scheduled to be released, giving yet another snapshot of where the race stands in Iowa.

"I am not nervous at all," Clinton said in an interview yesterday on NBC's Today show. "I'm excited about where we are."

Her actions speak otherwise.

At the last Democratic debate in December, Clinton barely acknowledged that Sanders was on the stage with her, except when responding to his criticisms. Clinton trained nearly all her fire on GOP front-runner Donald Trump.