US Democratic presidential front-runner Hillary Clinton is planning a massive advertising blitz to fend off the challenge from Senator Bernie Sanders, according to a report.

In the first three months of 2016, Clinton’s campaign would almost double the proportion of money it devoted to advertising last year, Reuters reported, citing a summary of Clinton’s spending plans.

“The significant early resources mobilized by our Finance Committee in 2015 helped us make smart early investments in key areas – and will enable us to continue with strategic allocations in Q1 of 2016," the campaign said.

The first quarter is a crucial stage in the US presidential race. It begins with the Iowa caucus on February 1, followed by the New Hampshire primary on February 9, and a number of early votes through March.

Clinton raised more than $112 million last year for the primary election, including $37 million in the fourth quarter.

Clinton’s campaign is facing a host of challenges with the Republican candidates raising questions about the past sexual affairs of her husband, former President Bill Clinton, as well as about her use of a private email sever to handle State Department business when she was secretary of state.

GOP front-runner Donald Trump has spent weeks criticizing Bill Clinton in his campaign events, calling the former president "one of the great abusers of the world" and “one of the great woman abusers of all time.”

Bernie Sanders (L) arrives prior to US President Barack Obama's State of the Union address in the House chamber of the US Capitol on January 12, 2016. (AFP photo)

The Federal Bureau of Investigation (FBI) has expanded its investigation of Hillary Clinton's use of a private email during her time as secretary of state.

The ongoing investigation is now focused on determining whether the possible “intersection” of Clinton Foundation affairs and State Department business may have violated public corruption laws, three intelligence sources told FOX News on condition of anonymity.

In addition, a congressional committee has launched a probe about the cyber-hack vulnerability of the email server Clinton used at home.

Texas Rep. Lamar Smith, chairman of the House Science, Space, and Technology Committee, announced he is seeking information from several private companies that provided Clinton with software and services for her server.

New national polling shows that Sanders is continuing to narrow the gap with Clinton, creating a sense of déjà vu about 2008, when Clinton was eventually defeated by underdog Barack Obama.

Sanders, who was behind by as many as 30 percentage points in the summer, now enjoys a comfortable lead in New Hampshire, with a 50-37 point margin, according to a Fox News poll released last week.

Meanwhile, Clinton’s commanding lead in Iowa is dwindling and the two rivals are locked in a tight race in the Hawkeye State.