As the pressure builds ahead of the next contest in the primary fight, ABC News recently obtained an audio recording of what sounds like intensive message polling conducted on Clinton's behalf to a caller in the northern part of the state.
In the recording, provided to ABC News by the Sanders campaign, a woman's voice asks a man on the other end for, among other things, his impression of both talking points that are favorable to Clinton as well as a series of attack lines against the Vermont senator.
The Sanders campaign said the recording and that type of polling are evidence that the Clinton campaign is nervous about a state they once thought they had locked up.
"I am not surprised at all that they are testing negative lines of attack," Tad Devine, a strategist with the Sanders campaign told ABC News in response to the audio recording.
"It is clear they are preparing for a much more negative campaign," said Devine, who was not the one who provided the recording to ABC News. "They are clearly seeing Nevada as close as we’re seeing it.”
The woman on the call first asks the listener to rate how well Clinton and Sanders would do in the general election, as well as with the various aspects of being president if elected.
The woman then reads negative statements the two candidates have said about each other and asks the listener, based on those statements, for whom he would vote.
She then pivots to gain the listener's reaction to pro-Clinton talking points.
"For each of these statements that I read, please tell me how much more likely it makes you to vote for Clinton as president,” the woman says. One of the statements she reads is, “Hillary Clinton says it is not enough to have plans that sound good on paper, but won’t ever actually happen. ... Does that statement make much more likely, somewhat more likely, a little more likely or no difference at all?”
She then tests lines about Clinton’s primary opponent. “Next, you’re going to hear some statements that someone could make about Bernie Sanders. After each one, please tell me how much they concern you,” she says. "Bernie Sanders is making big campaign promises that will cost up to 20 trillion dollars. The New York Times said his plans are not realistic. Other independent experts said his plans are unworkable and dead on arrival in Congress. That's the recipe for even more political gridlock ..."
The man on the other end, who the Sanders campaign says is a Sanders' supporter, says the statement does not concern him. The audio goes on with lines about Sanders’ voting history on gun manufacturing and sales, immigration and health care.
When ABC News presented the audio recording to an aide with the Clinton campaign, the aide said the campaign could neither confirm nor deny that the audio was from polling done by the campaign or an outside group affiliated with the campaign.
However, the aide argued that it was an example of “message-testing” polling, common among all campaigns.
“This is just the latest in a long, sad list of conspiracy theories from a Sanders campaign that can’t handle the scrutiny they’re now facing. Our campaign has not and does not do push polling,” Jesse Ferguson, a spokesperson for the Clinton campaign, told ABC News. The term push polling is highly charged in political circles and tends to refer to negative or biased polling that could sway the listener.
Sanders pollster Ben Tulchin told ABC News that in his opinion the poll recorded was more negative than anything he ran.
“It came out of desperation that their numbers are slipping in Nevada and they threw a bunch of stuff out there to see what they could use,” he added.
On Monday, the Clinton campaign stepped up its attacks on Sanders, sending a text message straight to Clinton supporters that pegged the Vermont senator as anti-Obama.
"Happy Presidents Day! Hillary is proud to stand with Pres. Obama today and every day. You might be surprised about someone who isn’t," the text message read.
Following the text message, supporters received a robocall from the campaign, including audio of Sanders calling for Obama to have a challenger in the 2012 election.
ABC News' Liz Kreutz contributed to this report.