U.S. Rep. Mazie Hirono, lifted by the strong support of core Democratic voters, has a double-digit lead over former U.S. Rep. Ed Case in the Democratic primary for U.S. Senate.

A new Hawaii Poll has Hirono ahead of Case 55 percent to 37 percent with 8 percent undecided only a few weeks before the Aug. 11 vote. The liberal Hirono’s strength is built among traditional Democrats, neighbor island voters and union households. Case, a moderate, does better among independents and Republicans.

Both Democrats retain a double-digit advantage in a potential contest against former Gov. Linda Lingle, the leading Republican, in the November general election.

The results are nearly identical to the last Hawaii Poll released in February, indicating that primary voters made up their minds early and have seen nothing through the campaign to change their minds. Many political analysts predict, however, that the gulf between the Democratic nominee and Lingle will close after the primary as more voters examine the campaign.

"I would call it a stagnant race," Rebecca Ward, president of Ward Research Inc., which conducted the poll for the Star-Advertiser and Hawaii News Now, said of the primary. "There hasn’t been any movement and, frankly, I’m not sure there is enough going on with the campaigns that we would expect much movement."

Hirono and Case stayed off the television airwaves during the spring and early summer and only began advertising in the past few weeks. Hirono, who has raised nearly five times as much money as Case, has saved up for a possible campaign against Lingle. The congresswoman’s ads have focused on her immigrant background, renewable energy, early childhood education and science, technology, engineering and math initiatives — themes directed at Lingle, not Case.

The Democrats have had four debates — a fifth, sponsored by Hawaii News Now and the Star-Advertiser, is set for tonight — but few significant differences have emerged that were not apparent from the start of the campaign to replace retiring U.S. Sen. Daniel Akaka, also a Democrat.

The poll was conducted from July 12 through Saturday. The telephone survey for the primary was taken among 606 very likely voters statewide and had a margin of error of 4 percentage points.

The general election comparisons and favorability ratings were taken among 756 very likely voters statewide and had a margin of error of 3.6 percentage points.

Overall, voters statewide have equally favorable opinions of Hirono and Case. But Hirono is viewed much more favorably among voters who usually vote Democratic, live on the neighbor islands, and are in union households, key demographics in a Democratic primary.

In the primary, Hirono leads Case 63 percent to 30 percent among traditional Democrats, 62 percent to 30 percent on the neighbor islands, and 59 percent to 33 percent in union households.

"It’s humbling to see this response to my candidacy, and it confirms the people of Hawaii are looking for a senator who understands the struggles our families are going through, has a plan to make Hawaii stronger and more sustainable with home-grown energy and local food, and brings a cooperative and collaborative approach to the U.S. Senate," Hirono said in an email.

Case, who describes Hirono as a far-left extremist, is up 54 percent to 36 percent among independents and 66 percent to 31 percent among Republicans who say they will vote in the Democratic primary. The poll found that 20 percent of primary voters intend to pull Republican ballots, which is higher than GOP turnout in recent primary elections, but a figure that could increase because of Lingle’s extensive advertising during the past several weeks.

Case is banking on higher-than-average voter turnout, particularly on Oahu, where former Gov. Ben Cayetano’s anti-rail campaign for mayor could motivate voters in East Honolulu and Windward neighborhoods where Case has previously done well. The poll has Hirono with a 10-point edge on Oahu.

Case said the Hawaii Poll is "light-years off" his recent internal poll, a Lingle internal poll and an automated telephone survey conducted by the online news service Civil Beat and Merriman River Group, which all have the primary essentially tied. He also cited a Ward Research poll in the primary for governor in 2002 that had Hirono over Case by 20 points. Hirono defeated Case by about 1 point.

"With our tremendous statewide grassroots support and promise of strong effective leadership, we believe we’re running even with Hirono with momentum in our favor in these final weeks," he said in an email. "We also believe that there are far more secret undecideds and supporters than reported in this or any other poll and that, as voters think their choice through, they will vote for us in the privacy of the voting booth. We further believe what most voters think — that at the end of the day, regardless of any poll, we will beat Lingle and Hirono won’t."

The poll found Hirono up over Lingle 58 percent to 39 percent and Case ahead of Lingle 56 percent to 38 percent. Lingle performs much better than both Democrats, however, among independent voters. Lingle’s campaign advisers acknowledge that the former governor must win among independents and attract moderate-to-conservative Democrats to compete.

But retired Maj. Gen. Robert G.F. Lee, Lingle’s campaign manager, said it is difficult to take the Hawaii Poll seriously because he believes it included too many voters — 62 percent — who said they usually vote Democratic. He said the number of independent voters should have been higher, given exit poll data from recent elections.

"As Hawaii’s voters continue to move toward the center, the polling data on the independent voters is the most significant portion of this poll," he said in an email. "Even though Ward Research only uses 19 percent independents, Gov. Lingle is winning that portion of the electorate with a 26 percent margin. These figures indicate that Gov. Lingle’s message of bipartisanship is resonating with the voters who will make a difference in this campaign: the moderates."

Ward said the poll’s sample reflected Hawaii’s history as a Democratic state. Many voters who describe themselves as independents in other surveys, for example, often vote Democratic. "This to me is consistent with everything we’ve known to be true about Hawaii politics," she said.