Wells Fargo & Co. has offered to by all of Wachovia Corp. for about $15.1 billion, trumping Citigroup Inc.’s government-assisted offer for part of the embattled Charolotte, N.C.-based lender.

The stock swap values Charlotte-based Wachovia at $7 a share, the companies said today in a joint statement. Wachovia traded at $6.92 in 8:54 a.m. Iowa time on the New York Stock Exchange, up 77 percent from yesterday. Wells Fargo shares rose 9.6 percent to $38.55 in 8:53 a.m. Citigroup fell 12 percent to $19.70, Bloomberg reported.

Wells Fargo, whose biggest stakeholder is billionaire Warren Buffett’s Berkshire Hathaway Inc., said its offer keeps Wachovia intact and needs no U.S. assistance.

Citigroup’s bid of $2.16 billion on Sept. 29 for Wachovia’s banking businesses — valued at about $1 share — relied on help from the Federal Deposit Insurance Corp. (FDIC) and left out the securities brokerage and Evergreen mutual fund units.

The FDIC said it stands behind the Citigroup deal, the Atlanta Business Chronicle reported.

The Wells Fargo offer “provides superior value compared to the previous offer to acquire only the banking operations of the company,” Richard Kovacevich, 64, chairman of San Francisco-based Wells Fargo, said in the statement. “Wachovia shareholders will have a meaningful opportunity to participate in the growth and success of a combined Wachovia-Wells Fargo.”

The stock swap gives Wachovia shareholders 0.1991 shares of Wells Fargo common stock for each share they own, allowing them to salvage some value after Wachovia’s 90 percent decline this year. Citigroup’s offer would have left the remnants of Wachovia, including the securities brokerage, to trade as an independent company.

As far as Citigroup is aware, its agreement with Wachovia is still in place, said a person familiar with Citigroup’s position. Regulators will review the Wells Fargo bid “to achieve an outcome that protects all Wachovia creditors, including depositors, insured and uninsured, and promotes market stability,” according to a statement from the Federal Reserve.

Wells Fargo expects charges related to the acquisition of about $10 billion, and the company said it would issue as much as $20 billion of new securities, mostly common stock, should the deal go through. Wachovia agreed to give Wells Fargo preferred stock that would represent 39.9 percent of Wachovia’s voting power.

Wells Fargo said it will acquire all of Wachovia’s businesses, preferred equity and banking deposits. Chief Financial Officer Howard Atkins said in the statement that the acquisition will add to earnings per share by the third year after completion and should produce an internal rate of return of at least 15 percent.