BlackRock, Exxon Mobil Corp’s second largest shareholder, is backing several of upstart hedge fund Engine No. 1’s candidates to join the company’s board, in a blow to the energy giant, according to people familiar with the matter.

With the annual shareholder meeting Wednesday, Exxon has been scrambling to win the backing of major institutional investors for its directors in its toughest boardroom battle in recent history. Exxon has faced mounting criticism for its reluctance to invest more in renewable energy and for years of weak financial performance.

BlackRock, the world’s largest fund manager, has a 6.7% stake in Exxon, and its vote illustrates how frustrated shareholders have become after years of having the company dismiss concerns about its strategy. Investors have grown much more serious about fighting climate change.