CHARLOTTE, N.C. — Bank of America said today it will buy Countrywide Financial for $4.1 billion in stock, a deal that rescues the country’s biggest mortgage lender and expands the financial services empire of the nation’s largest consumer bank.

The acquisition will make Charlotte-based Bank of America Corp. the nation’s biggest mortgage lender and loan servicer.

Bank of America said it initially plans to operate Countrywide separately under the Countrywide brand, with integration occurring no sooner than 2009.

The transaction represents a 7.5% discount to where Countrywide shares ended yesterday after they soared on news that a rescue plan was in the works. It also effectively leaves Bank of America with a big loss on its $2 billion August investment in Countrywide Financial Corp. during the height of the summer’s global credit crisis.

An aggressive dealmaker who has already snapped up behemoths FleetBoston Financial and MBNA, the Bank of America chief executive, Ken Lewis, this time isn’t buying a financial winner. Delinquencies and loans in pending foreclosure are rising in Countrywide’s loan portfolio, and Mr. Lewis said today “there are near-term challenges” in the nation’s housing market.

But Countrywide’s troubles have allowed Mr. Lewis to sweep in and add a major business line to his supermarket of financial products on the cheap.

“Countrywide presents a rare opportunity for Bank of America to add what we believe is the best domestic mortgage platform at an attractive price and to affirm our position as the nation’s premier lender to consumers,” Mr. Lewis said in a statement.

It also places Mr. Lewis in the position of a market savior. By buying Countrywide, he’s keeping the industry and regulators from the messy task of figuring out who would take on the responsibility of collecting payments for the 9 million American home loans serviced by the Calabasas, Calif.-based lender. Mr. Lewis said today there was no government support for Countrywide’s loan portfolio.

“There’s still plenty of risk involved,” a senior analyst at Celent, a Boston-based financial research and consulting firm, Bart Narter, said. “He’s brave to do it. But I think that it’s very likely down the road to be profitable, maybe not immediately, but long-term.”

There was no immediate work on job cuts, but analysts said they expect some among the ranks of Countrywide’s 15,000 employees. Mr. Lewis said he would like the Countrywide chairman and chief executive, Angelo Mozilo, to stay with the combined companies until the deal is done.

“Angelo has told me that he will do anything that we want him to do,” Mr. Lewis said. “I would guess that he’ll want to go have some fun. I will talk with him next week about his personal desires. Many of the senior people will have big operating roles in this company.”

Shareholders of Countrywide will receive 0.1822 of a share of Bank of America stock in exchange for each share of Countrywide. The deal is expected to close in the third quarter and to be neutral to Bank of America earnings per share in 2008 and lift earnings per share in 2009, excluding buyout and restructuring costs.

Bank of America expects $670 million in after-tax cost savings in the transaction, or 11% of the expense base of the two companies’ mortgage operations.

The agreement has been approved by both companies’ boards and is subject to regulatory and Countrywide’s shareholders approval.