Why is Hillary Clinton losing her lead?

Less than three weeks shy of the first primary election, Hillary Clinton is losing her once-sizable lead on Bernie Sanders, the self-proclaimed socialist senator from Vermont.

The latest smattering of polls have shown that Senator Sanders has the edge in Iowa and has widened his lead in New Hampshire. Nationally, Mrs. Clinton is still ahead, but the race is tightening. According to the latest New York Times/CBS News poll, she’s now only seven points ahead of Sanders nationwide, 13 points down from her lead in December.

In Tuesday’s Quinnipiac University poll, 49 percent of the nearly 500 surveyed likely Democratic caucus participants in Iowa said they support Sanders, and 44 percent went with Clinton. That's a shift from December, when Clinton was at 51 percent and Sanders at 40 percent. Meanwhile in New Hampshire, likely Democratic primary voters put Sanders ahead of Clinton by 14 points, 53 percent to Clinton’s 39.

One possible explanation for her diminishing edge is Sanders’s recent push with advertising. In December, his campaign aired more than 7,600 television ads – nearly one third more than Clinton’s ads. Since November, his campaign has spent $9.7 million to Clinton’s $7.4 million.

Another possibility involves Bill Clinton’s history of sexual misconduct. Thanks to Donald Trump, the former president’s past scandals have been dragged out from under the rug. And some conservative pundits believe it’s had a toll on Hillary Clinton’s polling.

“The only difference between mid-December and now is the Bill Clinton issue,” writes Dick Morris, a former adviser who has become a harsh critic of the Clintons, on The Hill.

Meanwhile, as The Atlantic’s David Graham points out, Sanders hasn’t faced any substantial attacks from Republicans and his visibility has increased.

“The Sanders campaign sees the same dynamic from a different perspective,” Mr. Graham writes, “which also relies on the idea that voters are just tuning in. They say that this is proof that Sanders’s electability and message about inequality is resonating with voters now.”

But, according to Clinton’s camp, the slouching numbers aren't cause for serious concern.

"I’m working hard and I intend to keep working as hard as I can until the last vote or caucus-goer expresses an opinion. I’m excited about where we are,” the former secretary of State said on NBC's "Today" show Wednesday morning.

“I’m not nervous at all,” she added.

Political experts have long pointed to the theory of “natural tightening,” in which the race invariably grows closer toward the end, when more and more voters are paying attention. Historically, the weeks leading up to the primaries are fraught with inconsistent rankings. One candidate could be in the lead one day, and another the following week. In turn, polls right before the first votes are cast are not accurate in predicting who will ultimately win the nomination.

To the Clinton campaign, this is a viable explanation.

"Since the campaign started, we have said this race will be a competitive, tough race that would tighten and we’d have to earn the nomination,” spokesman Jesse Ferguson said in a statement. "We have built a tremendous grassroots organization in Iowa fueled by enthusiasm for Hillary Clinton and her record, that is set to compete and win.”