By STEPHEN LABATON and MIGUEL HELFT

An expensive legal and political campaign last year by Microsoft helped delay completion of Google’s $3.1 billion bid for the online advertising company DoubleClick. Microsoft filed briefs against the deal in the United States and abroad, testified against it in Congress, and worked with a public relations firm to generate opposition.
Now Google is preparing to strike back.
With Microsoft bidding nearly $45 billion to buy Yahoo, Google has begun to lay the groundwork to try to delay, and possibly derail, any deal. Google executives have asked company lobbyists to develop a political strategy to challenge the acquisition, which could threaten Google’s dominance of Internet advertising. Google’s top legal officer posted a statement Sunday that criticized the proposed deal.