Bernie Sanders' challenge to Hillary Clinton is picking up steam as the Vermont senator leads the Democratic presidential front-runner by 9-percentage points in New Hampshire, according to the latest poll to come out of the key early-primary state.

The longest-serving independent in U.S. congressional history has the support of 41 percent of Democratic voters in Granite State, while Clinton logged only 32 percent, NBC reported based on the survey it conducted in collaboration with the Marist Institute for Public Opinion.

Vice President Joe Biden, who continues to ponder a White House bid, came in at 16 percent, and no other Democrat received more than 1 percent backing. If Biden was to decide not to join the race, Sanders' lead over Clinton, in the current survey, would increase to 11 points -- 49 percent to 38 percent.

The poll was conducted between Aug. 26 and Sept. 2, among 356 potential Democratic primary voters; it had a margin of error of approximately 5 percentage points.

In the first-in-the-nation caucus state of Iowa, meanwhile, Clinton is still in the lead, but Sanders is gaining ground on the former secretary of state, The Hill noted. The former secretary of state logged 38 percent support in the Hawkeye State, while the self-proclaimed socialist senator was backed by 27 percent of respondents. Biden, finally, was favored by 16 percent of Democrats.

The Iowa poll was conducted during the same time frame as the one in New Hampshire and featured a sample size of 345 potential Democratic caucus-goers; its margin of error was approximately 5 percentage points, as well.

Clinton's slump in the critical early-primary states is largely due to the controversy surrounding her use of a private email account during her four-year tenure as the nation's top diplomat, CNN analyzed.

"The 'drip, drip, drip' nature of both congressional and federal inquiries into her email practices are having an effect on her candidacy," eroding voters' trust in the Democratic front-runner, CNN commented.