The list of signers is far from a listing of 88 current Fortune 500 CEOs. Some 40 are “formers,” reflecting the fact that sitting business leaders need to be careful in being too explicitly on one side or the other of America’s deepening political divide. Others, like former Treasury Secretary Larry Summers, are from academia more than from the business world. While the letter reveals that many business leaders are supportive of Harris, it does not suggest that the whole business world is on her side. Donald Trump has his own coterie of business leaders on his side, including Elon Musk, Steve Schwarzman, Bill Ackman, and Marc Andreessen.

Traditional affinities

There has long been political diversity among business leaders. Financier JP Morgan generally—and generously—backed Republicans throughout his career. Yet in 1884, he backed the conservative Democrat Grover Cleveland for president. Standard Oil’s John D. Rockefeller habitually gave money to Republicans but still clashed with trust-busting Republican Teddy Roosevelt, who criticized Rockefeller’s monopoly and initiated policies that led to the breakup of his company.

Many of the Harris endorsement letter signers are from the worlds of technology and entertainment. Connections between Democrats and Hollywood executives go back a long time. Harry and Jack Warner of the Warner Brothers studio strongly backed Franklin Roosevelt for president in 1932. The brothers attended his 1933 inauguration and regularly made films that advanced Roosevelt and his causes. In later years, agent and then studio head Lew Wasserman befriended Democratic president Lyndon Johnson and practically created the pipeline of Hollywood cash going to Democratic politicians.

The close links with Hollywood and Democrats of course continue today as well. When Hollywood executive—and Harris letter signer—Jeffrey Katzenberg made it known that the money was drying up for Joe Biden‘s faltering presidential campaign, it was like a stake through the heart of the Biden candidacy. Silicon Valley, another source for many of the letter’s signers, is a newer industry, but it has also long been a Democratic stronghold.

A shifting landscape

The letter’s signatories are also indicative of the evolution of our economy away from manufacturing and industrial companies that have tended to favor Republican policies on regulation. White-collar industries like entertainment, media, and tech not only favor many Democratic policies, but they also have highly Democratic workforces that push back against executives who back Republican politicians.

In addition to external factors that may have eroded a perceived GOP edge among corporate leaders, Republicans need some introspection. Republicans have long been better for business in terms of their political rhetoric. Democrats are and have been regular bashers of corporations. However, in recent years we have been seeing more anti-business rhetoric from Republicans as well. Democratic corporate bashing is still more common and more critical, but anti-business rhetoric from Republicans erodes a traditional area of Republican advantage.

Beyond the rhetoric, Democrats have long backed interventionist antitrust policies, while Republicans have been more enamored of the theories of Robert Bork, who argued that big is not bad as long as consumers benefit. In recent years, though, an increasing number of Republicans have sided with modern trustbusters like Federal Trade Commission head Lina Khan and said they want to break up big companies, particularly in the tech space.

The final factor is in terms of personal relationships. While Dwight Eisenhower felt close to “the gang,” his group of corporate executive golf-playing buddies, modern-day executives, especially in the tech and finance worlds, are more likely to find affinity with Democratic politicians. They will frequently have gone to the same schools, live in the same blue-bubble cities and suburbs, and share the same cultural interests. From many business leaders’ perspectives, Democrats may bash them, but as long as Republicans are bashing them as well, they might as well support the people that their kids go to school with.

The relationships between the main political parties and big business have long been complex—and are even more complex in our current era. But the pro-Harris letter reveals that Republicans have some work to do if they want to be seen as the pro-business party in future elections.

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The opinions expressed in Fortune.com commentary pieces are solely the views of their authors and do not necessarily reflect the opinions and beliefs of Fortune.