SAN FRANCISCO — In a preview of what could shape up to be a fierce antitrust fight over Microsoft’s $45 billion offer for Yahoo, Google’s chief legal officer on Sunday made clear that the search giant would not sit on the sidelines, questioning whether Microsoft would “now attempt to exert the same sort of inappropriate and illegal influence over the Internet that it did with the PC.”

Writing on the official Google blog, David Drummond, the company’s senior vice president for corporate development and chief legal officer, argued that a combined Microsoft and Yahoo (YHOO) “raises troubling questions” and would pose significant competitiveness issues. “Could the acquisition of Yahoo allow Microsoft — despite its legacy of serious legal and regulatory offenses — to extend unfair practices from browsers and operating systems to the Internet?” he wrote.

Drummond sketched a future where a MicroHoo controls an “overwhelming share of instant messaging and web e-mail accounts.”

“Could a combination of the two take advantage of a PC software monopoly to unfairly limit the ability of consumers to freely access competitors’ e-mail, IM, and web-based services?” he asked. Making clear that he was directing that rhetorical exercise at regulators in Washington, D.C., and elsewhere, he wrote, “Policymakers around the world need to ask these questions — and consumers deserve satisfying answers.”

Repeatedly calling the software giant’s bid on Friday for an iconic Silicon Valley company “hostile,” Drummond portrayed the Internet itself as at stake. “This is about more than simply a financial transaction, one company taking over another,” he wrote. “It’s about preserving the underlying principles of the Internet: openness and innovation.”

Of course, as Google has expanded its hold over the Internet through its dominance of search and online advertising, observers have questioned whether the Mountain View, Calif., company itself poses a Microsoftian threat to online openness. In fact, on Friday a number of Silicon Valley movers and shakers welcomed the Microsoft offer for Yahoo as a potential counterweight to Google’s ambitions.

In his blog post, Drummond also seemed to signal Google’s interest in a white knight riding to its Silicon Valley rival’s rescue. “We believe that the interests of Internet users come first — and should come first — as the merits of this proposed acquisition are examined and alternatives explored.”

The Wall Street Journal and New York Times are reporting that Google CEO Eric Schmidt called Yahoo chief Jerry Yang on Friday to offer help in fending off Microsoft’s takeover bid.

A Google spokesperson said that the company would have no further comment “for the time being.”