Bernie Sanders' Popularity Making Hillary Clinton Campaign 'Nervous'

BOSTON -- The Clinton campaign appears to be getting nervous.

While from the start Hillary Clinton's campaign has said they were not taking anything for granted, the Democratic frontrunner didn't always act that way -– for months carrying on as though her challenger, Bernie Sanders, didn’t exist.

Over time, however, the 74-year-old Vermont senator has risen in the polls and gained growing support that eventually became too big for Clinton to ignore.

Now, just a few weeks until the first voting begins in Iowa and New Hampshire, her campaign is showing signs they're more anxious than ever about the outcome of these races.

The most blatant sign came earlier this week when Clinton's campaign manager Robby Mook sent a fundraising email to supporters with the subject line "nervous."

"There's a situation developing in Iowa and New Hampshire that could change the course of this election," Mook wrote in the e-mail, noting that Sanders' campaign is outspending Clinton in TV ads in the two early states.

Two days later, the campaign sent another fundraising email mentioning the "(seriously!) tight" polls in New Hampshire.

The e-mail came after Fox News released a poll showing Sanders, who is from neighboring Vermont, ahead of Clinton in New Hampshire by 13 points.

In light of tightening polls, the Clinton campaign has also ramped up its aggressive opposition against Sanders.

On Monday, the campaign pro-actively went after Sanders ahead of his Wall Street reform speech. On Tuesday and Wednesday, Clinton questioned his electability. On Thursday, the campaign engaged in a back-and-forth with him on their paid leave plans. And on Friday, the campaign went after him on guns after Sanders' campaign said there is "zero daylight" between him and Obama on gun control.

"You know, maybe it's time for Sen. Sanders to stand up and say I got this one wrong," Clinton said during a rare, last-minute phone interview on "Hardball" with Chris Matthews on Friday night.

In addition, the Clinton campaign has launched Bill Clinton out on the trail this week. On Wednesday, he's expected to go back to New Hampshire where he'll be holding campaign events exclusively near the Vermont border -– a part of the state where Sanders signs are very prevalent.

On Saturday, the campaign released a new television ad that will air in Iowa and New Hampshire, with the message that Clinton will be more effective than Sanders going after the GOP.

Asked about whether their increased engagement with Sanders is a sign they're concerned about Sanders, Clinton campaign spokesman Brian Fallon said: "Republicans are trying to rip away the progress we have made on issue after issue, and we are making clear that Hillary Clinton is the best candidate to take them on and get things done."

But the Sanders campaign has taken it as a sign of worries.

"See the new Fox poll that had Bernie up a lot in New Hampshire? No wonder they're in attack mode," Sanders communications director Michael Briggs said in a statement. "Secretary Clinton and her team are getting nervous and nasty because the so-called inevitable nominee anointed by the establishment eight months ago doesn't look so inevitable anymore."