“Bigger is better and biggest is best,” in Mr. Lewis’s worldview, Ms. Bush said.

Even so, the Merrill deal is laced with some contradictions. Only a year ago, Bank of America appeared to have given up on investment banking after suffering extensive losses in a business in which it had little experience.

Mr. Lewis had spent more than $625 million to expand into investment banking, only to see all of its trading businesses soaked in red ink. A few months ago he said at a conference that he would not spend “petty cash” for an investment bank.

Apparently, Mr. Lewis was willing to forgo those misgivings because he saw a Merrill takeover as a once-in-a-lifetime opportunity after years of a frustration trying to build an investment bank from scratch.

The deal would vastly expand Bank of America’s reach into equities and emerging markets. It strengthens its roster of top investment banking executives. And it gives the bank Merrill’s brand and its “Thundering Herd” of 16,000 brokers to help push its credit cards and loans.

“This was almost a perfect fit, and we thought it was close enough to the bottom that we could make the deal work and be very good for our shareholders,” Mr. Lewis said. “I don’t think it will come out of the doldrums in the next week or so, but as soon as the market gets its legs again, you will see a higher value.”

The stock market is more hesitant, concerned that Mr. Lewis may have moved so quickly that he could eventually find himself exposed to more troubled Merrill assets than he bargained for  in addition to the risky mortgage portfolio he inherited from Countrywide.

Mr. Lewis said he was confident that the deal was the right move, but conceded possible hurdles in the near term.