UNITED STATES - JANUARY 14: Sen. Brian Schatz, D-Hawaii, speaks during a news conference in the Capitol to announce the newly formed Senate Climate Change Task Force. (Photo By Tom Williams/CQ Roll Call)

A new poll suggests that a Democrat facing a credible primary challenge may have gained some momentum in his bid to be reelected to the Senate.

The left-leaning firm Public Policy Polling released a poll Tuesday showing Sen. Brian Schatz (D-Hawaii) with a commanding 15-point lead over his primary rival, Rep. Colleen Hanabusa (D-Hawaii). The poll was conducted on behalf of the progressive group Democracy for America, which came out in support of Schatz in February.

Schatz was appointed by Hawaii's Gov. Neil Abercrombie (D) to replace Sen. Daniel Inouye (D-Hawaii) after Inouye's death in December 2012. Schatz and Hanabusa will face off for the Democratic nomination in August.

Forty-nine percent of respondents told Public Policy Polling that they'd vote for Schatz, while only 34 percent said they'd pick Hanabusa. Fifty-five percent of those polled said they approved of the job Schatz is doing as senator.

Earlier this year, polling had showed a much smaller margin between the two candidates. Some of Schatz's gains could be attributed to a bump from President Barack Obama, who endorsed the senator in March.

Schatz has picked up a number of other endorsements from progressive groups like MoveOn.org, the Sierra Club, the League of Conservation Voters and the Progressive Change Campaign Committee. Hanabusa, for her part, has the backing of EMILY's List, a group that works to elect Democratic women who support abortion rights.