To compete against Democratic rival Hillary Clinton, Republican frontrunner Donald Trump has finally conceded, he needs more than the bare-bones campaign team that led him to primary success. Yet he’s finding that many of the GOP’s most experienced political aides just aren’t willing to work for him.From Texas to New Hampshire, well-respected members of the Republican Party’s professional class say they can’t look past their deep personal and professional reservations about the presumptive presidential nominee.

Many fear a stain on their résumés

There are exceptions, but many operatives who best understand the mechanics of presidential politics fear that taking a Trump paycheck might stain their résumés, spook other clients and even cause problems at home. They’re also reluctant to devote months to a divisive candidate whose campaign has been plagued by infighting and disorganization.“Right now, I feel no obligation to lift a finger to help Donald Trump,” said Brent Swander, an Ohio-based operative who has coordinated nationwide logistics for Republican presidential campaigns dating back to George W. Bush.

‘He is bullying, demeaning others’

“Everything that we’re taught as children not to bully, not to demean, to treat others with respect everything we’re taught as children is the exact opposite of what the Republican nominee is doing. How do you work for somebody like that? What would I tell my family?” Mr. Swander said.Mr. Trump leapt into presidential politics with a small group of aides, some drafted directly from his real estate business, with no experience running a White House campaign. An unquestioned success in the GOP primaries, they have struggled to respond to the increased demands of a general election as Mr. Trump emerged as the party’s presumptive nominee.

Decline invitations to work for him

As in years past, the primary season created a pool of battle-tested staffers who worked for other candidates, from which Mr. Trump would be expected to draw. But hundreds of such aides have so far declined invitations to work for Mr. Trump.They include several communications aides to Chris Christie, as well as the New Jersey Governor’s senior political adviser, Michael DuHaime, who said he declined direct and indirect inquiries to work for the billionaire.

Trumped him, en masse

Chris Wilson, a senior aide to Ted Cruz, said the Texas senator’s entire paid staff of more than 150 ignored encouragement from Trump’s team to apply for positions after he dropped out of the presidential race. Mr. Wilson said that even now, many unemployed Cruz aides are refusing to work for the man who called their former boss “Lyin’ Ted.”

That’s the case for Scott Smith, a Texas-based operative who travelled the country planning events for Mr. Cruz, having previously worked on presidential bids for Mr. Bush and Texas Gov. Rick Perry.“It’s very clear that none of us are going to work for Trump,” Mr. Smith said. “Even if I wanted to work for Trump, my wife would kill me.”

Campaigns cut into family time

Mr. Smith, as did many experienced operatives interviewed for this story, noted the intense personal sacrifice required of presidential campaigns. Many staffers don’t see their families for long stretches, work long hours on little sleep and enjoy no job security.With Mr. Trump, Mr. Smith said, “I would feel like a mercenary. I can’t be away from my young children if it’s just for money.”

But he needs manpower

Mr. Trump’s need for additional staff is acute. His paltry fundraising network brought in less than $2 million last month. He has just one paid staffer to handle hundreds of daily media requests and only a handful of operatives in battleground states devoted to his White House bid.

Last month, Mr. Trump fired Rick Wiley, former Scott Walker campaign manager he brought on to run his nationwide get out the vote effort. On Monday, Mr. Trump fired campaign manager Corey Lewandowski, who conceded he lacked the experience needed to grow Mr. Trump’s operation.“This campaign needs to grow rapidly … two, three, four hundred people in the next 90 days or less,” Mr. Lewandowski told the Fox News Channel. “That’s a hard job and candidly I’ve never grown something that big.”

But here is a hope

Campaign spokeswoman Hope Hicks did not respond to multiple requests for comment about the campaign’s hiring, but former adviser Barry Bennett downplayed any staffing challenging, suggesting the campaign should be able to double its staff by the party’s national convention next month.Mr. Trump announced four new hires in the past week to supplement a staff of around 70. That’s compared to Ms. Clinton’s paid presence of roughly 700, many of them well-versed in modern political strategy.

His senior team belongs to the past

Mr. Trump’s senior team, including campaign chief Paul Manafort and newly hired political director Jim Murphy, largely represent an older generation of political hands more active in the 1980s and 1990s. The campaign’s new Ohio director, Bob Paduchik, led state efforts for Mr. Bush’s 2000 and 2004 campaigns.

A new generation of top talent active in more recent years has shown little interest in Mr. Trump. In swing-state Iowa, experienced operative Sara Craig says she won’t work for Mr. Trump or even support him. “I am more interested in working on down-ballot races,” said Ms. Craig, who helped elect freshman Joni Ernst to the Senate and directed a pro-Bush super PAC.

Bad-mouthing keeps off others

Mitt Romney veteran Ryan Williams said he’s happy in his current consulting firm, where he’s involved with various other elections across the country, as well as with corporate clients.“When you sign up for a campaign, you’re putting your name on the effort. Some of the things that Trump has said publicly are very hard for people to get behind,” Mr. Williams said.