President Trump’s executive orders seeking to punish big law firms have led some of them to acquiesce to him and left others reluctant to take on pro bono cases that could put them at odds with the administration.

But as opponents of the White House’s policies organized to fight Mr. Trump in court on a vast range of actions and policies, they quickly found that they did not need to rely on Big Law. Instead, an army of solo practitioners, former government litigators and small law firms stepped up to volunteer their time to challenge the administration’s agenda.

“I don’t know if the administration knew how many little guys are out there,” said Michael H. Ansell, a solo practitioner in Morristown, N.J., who earlier this year joined the Pro Bono Litigation Corps, newly launched by Lawyers for Good Government, a legal nonprofit. He answered the nonprofit’s plea for lawyers willing to give at least 20 hours a week to an upcoming case. More than 80 volunteered.

Earlier this year, Mr. Trump issued a flurry of executive orders and implicit threats targeting major law firms whose past work or clients he did not like. While some of the firms chose to fight the orders and have so far prevailed in court, others hastened to settle, agreeing in the process to steer about $1 billion in pro bono work toward the administration’s favored causes. Even some of those who stood up to Mr. Trump have been leery of further provoking his ire by taking on cases opposing the administration.

But lawyers like Mr. Ansell have been eager to jump in. He helped interview plaintiffs for a lawsuit filed in U.S. District Court in the District of Columbia late last month against the Environmental Protection Agency. About 20 community, environmental and tribal groups, as well as the cities of Springfield, Mass., San Francisco and Sacramento are suing to restore money they were awarded but lost after the E.P.A. terminated its Environmental and Climate Justice Block Grants program.

Mr. Ansell typically handles small business disputes, “so I don’t have to worry about losing any big-time government contractor clients,” he said.