Travels and extensive conversations with Mr. Trump in recent weeks show that, while he is slowly beginning to bend to some candidate norms—opening state offices, readying ballot-access drives and preparing a tax plan—he continues to resist the experts’ view that he needs a conventional campaign apparatus.

“A lot of what I’m doing is by instinct,” Mr. Trump said in one of several interviews. “I assimilate a lot of information…and I believe in being strategic.” Instead of surrounding himself with what he called “political hacks,” Mr. Trump said, “I don’t need an inner circle.” His rationale: In an “age of specialization, I am tapping phenomenal people in every field.”…

Without much staff, campaign manager Corey Lewandowski has worked with outside advisers to flesh out the details of the tax plan, particularly to end up with a revenue-neutral result that neither raises nor lowers overall receipts. “We’re running an efficient organization with a business mind-set,” Mr. Lewandowski said. “We don’t need high-priced staff or consultants when leading authorities are volunteering to help Mr. Trump.”…

Republican strategist Kevin Madden said Mr. Trump faces new and serious threats. “One big question is whether he can turn momentum from celebrity fandom into an actual infrastructure to organize voters in cold gymnasiums in the dead of winter in Iowa…Trump hasn’t seen a full-on assault, which is just beginning, with millions of dollars in paid advertising and their relentless attacks.”