For months, European business leaders have watched as Donald J. Trump paved a path back to the White House by doubling down on an “America First” economic policy, putting protectionism and business-friendly tax and regulatory pledges high on the agenda.

With the U.S. presidential inauguration less than two months away, they are now furiously lobbying policymakers in Brussels with an appeal of their own: Put Europe First.

Nothing has galvanized the European business community quite as urgently as the prospect of a second Trump presidency, this time with Mr. Trump wielding bolder promises to disrupt the global economic order in ways that threaten to leave Europe lagging even further behind.

At a business conference in Paris this week, hundreds of corporate chiefs and lobby groups from France, Germany and Italy gathered to discuss just that. They described Mr. Trump’s election as a “wake-up call” for Europe to get its house in order — or face a painful economic fallout.

Part of their strategy is to increase investment across the European Union in areas like defense, technology and green energy, as well as to loosen regulation and link the trade bloc with common debt and a single capital market.