“Silicon Valley is stepping up,” said Sam Altman, who runs the valley’s most prominent start-up incubator, Y Combinator. “The companies are working on three fronts: They are vociferously objecting to the Trump policies they think are bad, they are trying to engage with him to influence his behavior, and they are developing new technology to work against policies and political discourse they don’t support.”

It is an improvised and complicated strategy. The companies are among the richest and most popular of American brands, which means they have a good deal of leverage. Yet they are also uniquely vulnerable — not only to presidential postings on Twitter and executive orders, but to the sentiments of their customers and employees, some of whom have more radical ideas in mind.

Many of the companies initially placed their bets on engagement after an upbeat meeting with the president-elect in December. That modest approach, which even the most risk-averse executive can endorse, showed its limits last week. After widespread customer defections, Travis Kalanick, the chief executive of Uber, was forced to step down from one of the administration’s advisory councils.