Washington (CNN) Jeb Bush narrowly leads the field in the state set to host the first primary of the 2016 presidential campaign, but Donald Trump's gains in the state suggest the billionaire businessman is establishing a following in New Hampshire.

As the low numbers at the top of the pack indicate, the field is far from settled. Twenty-one percent say they don't know which of the 19 candidates tested in the poll they'd support, and overall, 75% say they're not committed to any candidate.

Trump's gains seem connected to perceptions of him as a successful leader on economic issues. Trump easily tops the field when asked which candidate voters think can best handle the economy; 29% choose Trump, 13% Bush, with everyone else at 7% or less. And Trump is the clear leader among Republicans when they are asked which candidate would best handle international trade (27% choose Trump, 14% Bush).

Assessing the field's personal traits, Trump narrowly edges Bush at the top of the list when voters are asked which Republican candidate is the strongest leader (17% say Trump is, 15% Bush), and a whopping 46% say Trump is the one in the field "least likely to act like a typical politician if elected president." No one else comes close on that measure.

Illegal immigration emerges as the one issue tested in the poll on which voters prefer someone other than Trump or Bush. On that measure, 16% prefer Rick Perry's approach, Trump and Bush tie for second at 13% and Rubio is just behind at 11%.

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Trump's weakness may come from perceptions of his electability. While 37% say Bush is the candidate with the best chance to beat the Democratic nominee next year, just 7% say Trump has the best chance to win. Likewise, just 8% think he best embodies the personal characteristics and qualities a president should have, 19% think Bush does.

And Trump tops the list when New Hampshire GOP voters are asked which of the possible candidates they would not support under any circumstance, 23% have ruled him out entirely. Moderates are most apt to say they wouldn't back Trump, 31% say he's definitely not their guy, compared with 19% of self-described conservatives.

Overall, 10% say they would never consider Jeb Bush, and 10% say the same about Chris Christie. The rest of the field is under 5% on this question.

The good news for Bush in this poll comes from his favorability rating. Since announcing his candidacy for President, Bush's favorability rating has ticked upward and 50% now hold a positive impression of the former Florida governor. In the May Granite State Poll, 45% had a favorable view.

Trump too has gained since his campaign launch, though more still hold an unfavorable impression than a favorable one. In the new poll, 38% have a favorable view of Trump, 48% a negative one. In May, those figures were 27% favorable, 56% unfavorable.

Carly Fiorina joins these two as one of the few in the field who gained: Her favorability rating is up 10 points since May to 39%, with just 15% holding an unfavorable opinion of the businesswoman.

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New Hampshire voters seem to be souring on the rest of the field. Chris Christie, expected to formally enter the race next week, has seen his favorability drop 8 points since May to 31%, his lowest read in Granite State Poll trend back to 2013.

Lindsey Graham's ratings have dipped since he formally entered the race on June 1, with just 17% having a positive impression in the new poll, down from 25% in May. At the same time, his unfavorable rating has grown to 40%, up from 29% in May.

Ted Cruz, Mike Huckabee, Rand Paul, Marco Rubio, Rick Santorum and Scott Walker have all lost ground on their favorability ratings as well, making Fiorina's and Trump's increases more notable.

The CNN/WMUR New Hampshire Primary Poll was conducted by telephone by the University of New Hampshire Survey Center from June 18-24 among 1,010 adult residents of New Hampshire. Of those, 402 said they plan to vote in the Republican presidential primary. Results among likely Republican primary voters have a margin of sampling error of plus or minus 4.9 percentage points.