And lobbyists are just the latest major fundraising group not to open their wallets for the presumptive nominee.

The three most generous sectors to Mr. Romney’s campaign—Wall Street, health companies, and lawyers and lobbyists—have given little to the New York businessman so far, while those three sectors have poured more than $43 million into Mrs. Clinton’s campaign through May, according to the nonpartisan Center for Responsive Politics. Mr. Trump has recently begun ramping up his fundraising after largely self-financing the primary.

Lobbyist support is at least as much about fundraising as it is about spreading the word. They are connected to a wide range of corporate clients who both have deep pockets and carry plenty of influence.

Some lobbyists say they don’t support Mr. Trump and wouldn’t raise money if he asked them to. Others say they’re burned out, having raised millions earlier this year for candidates like former Florida Gov. Jeb Bush and Florida Sen. Marco Rubio.