It is the latest sign that businesses are ratcheting up lobbying against tariffs that the Trump administration has imposed, or is considering, as Mr. Trump says he will defend American manufacturing jobs. As of June 30, nearly 450 entities employed lobbyists on trade issues—up from about 160 at the start of the year and about 100 when Mr. Trump took office, according to lobbying-disclosure reports compiled by the nonprofit Center for Responsive Politics.

Few policy fights have triggered as big a jump in lobbying activity, although there are more lobbyists overall engaged on perennial issues such as taxes and health care. Some businesses are concerned about rising costs of imported materials; others, particularly farmers, about retaliatory tariffs imposed by China and Europe on U.S. exports.

At the Iowa State Fair last month, a lobbying group backed by the American Farm Bureau handed out “I Support Free Trade” buttons and urged farmers to sign posters proclaiming their opposition to tariffs.

Car manufacturers, auto dealers and vehicle parts makers together plan to run a campaign opposing new tariffs on the industry. And last week, the trade association for retailers including Target Corp. and Walmart Inc. brought 150 small retailers to meetings with lawmakers to talk about how tariffs could hurt their businesses.

“Every trade group is much busier because there’s a lot more activity across all aspects of what trade groups do,” said Steve Orava, who leads the international trade practice at law firm King & Spalding in Washington.

Not all industry groups oppose Mr. Trump’s tariffs. The National Cattlemen’s Beef Association, which represents U.S. ranchers and beef producers, backs the president’s tough trade stance. “We support the president’s overall goal of tearing down trade barriers; we support trying to take them on,” said association spokesman Max Moncaster. China and the European Union currently ban imports of U.S. beef raised with hormones.

And some industries benefit from import duties. Domestic steel companies support Mr. Trump’s tariffs on foreign steel, which have boosted prices and profits.

But most trade-focused lobbying this year has been against tariffs. When the Office of the U.S. Trade Representative took testimony on proposed tariffs in August, a majority of the industry representatives who participated said tariffs would hurt their businesses.

In a letter they plan to send to Congress on Wednesday, business groups will announce their latest effort to make the case against tariffs. The group’s multimillion-dollar Tariffs Hurt the Heartland campaign aims to tell the stories of farmers and business ownersdinged by import duties.

“Every sector of the U.S. economy stands to lose in a trade war,” said Matthew Shay, president of the National Retail Federation. The goal of the campaign is to “ensure Washington understands the real-world consequences of a trade war.”

The U.S. Chamber of Commerce, the National Association of Manufacturers, the Business Roundtable and the Koch brothersare running their own lobbying efforts to promote free trade.

The Trump administration is expected soon to impose tariffs on $200 billion of Chinese imports, on top of tariffs already in effect on $50 billion in goods from China. Mr. Trump has suggested even more duties are in the offing.

The U.S. has also placed tariffs on steel and aluminum imports and is conducting trade negotiations with Europe, Mexico and Canada. China, the EU and other trade partners have announced tariffs of their own on American goods.

The unusual mechanism Mr. Trump is using to impose the tariffs has meant that many lobbyists can’t rely on the usual playbook. For most big policy changes in Washington, such as last year’s tax bill, Congress writes and votes on legislation, a drawn-out process that gives industries many opportunities to weigh in.

In this case, Mr. Trump is using an obscure part of trade law that permits him to impose tariffs unilaterally, sometimes in the name of national security. That is why many of the industries seeking to roll back or avoid tariffs are targeting the Trump administration alone.

Earlier this year, comedian Ben Stein starred in ads calling tariffs “B-A-D economics.” The ads, sponsored by retail lobbying group the National Retail Federation, ran on a favorite show of Mr. Trump’s, “Fox & Friends” on Fox News.

Farmers for Free Trade, hoping to catch Mr. Trump’s eye, has run its ads mostly in Washington, as well as in the Palm Beach, Fla., media market when Mr. Trump is staying at his Mar-a-Lago resort there. The group also has identified 10 states that will be important to Mr. Trump’s re-election and is highlighting stories of farmers who would be hurt by his trade policies.

When Commerce Secretary Wilbur Ross last month visited Fargo, N.D., to discuss the impact of tariffs, the farmers’ group greeted him with a string of roadside billboards that read: “Secretary Ross, Tariffs Hurt ND Farmers.”

The Maine Lobster Dealers Association is agitating, too, saying tariffs will hit them harder than others because reciprocal tariffs imposed on the lobsters they sell to China don’t apply to lobsters sold in China by Canadian lobstermen, even though the lobsters are harvested from the same Atlantic waters.

“These guys want to sell lobsters, they don’t want to be wasting their time lobbying members of Congress,” said Annie Tselikis, the executive director of the Maine Lobster Dealers’ Association.