Support for Exelon Corp.'s (EXC) hostile bid for NRG Energy Inc. (NRG) slipped among NRG shareholders, with only 12% tendering their shares as of Tuesday evening.

Exelon reported the new figure Wednesday as it announced it would extend its exchange offer for NRG Energy shares to Aug. 21 from June 26. Exelon is extending the offer after NRG Energy's annual meeting scheduled for July 21, which is seen as a key event in its $6.2 billion takeover bid.

While a deadline extension was expected, the size of the drop off in NRG Energy shareholder support was surprising, said Brandon Blossman, an analyst at Tudor, Pickering, Holt & Co.

Exelon had won the support of more than 51% of NRG Energy's shareholders by a February deadline. The company attributed the pullback in support to timing, with Exelon extending the offer ahead of next week's deadline.

"Investors typically withdraw tendered shares between expiration dates, and do not re-tender until very close to the next scheduled expiration date," William Von Hoene, executive vice president and general counsel at Exelon, said in a statement. "Because the exchange offer was extended early due to NRG's delay in setting its annual meeting, that cycle could not play out here."

NRG Energy had no immediate comment on the announcement.

Exelon last fall launched an exchange offer to pay a fixed ratio of 0.485 Exelon share for each NRG share in a bid to create the largest power generator in the U.S. by output. NRG Energy's board has stiffly resisted the bid, saying it undervalues the Princeton, N.J., power plant operator.

The Chicago power company is facing growing pressure to raise its offer as shares of NRG Energy trade near the exchange offer. As of Tuesday's market close, the proposed deal represented less than a 4% premium for NRG Energy shareholders. The spread has narrowed considerably in recent weeks, with the premium standing at more than 15% a month ago. The exchange offer represented a 37% premium for NRG Energy shareholders when Exelon announced its bid in October.

Analysts have said a variety of factors, including NRG Energy's progress on a new nuclear project and its acquisition of Reliant Energy Inc.'s (RRI) retail business, require a higher offer. But the likelihood Exelon will have to raise new equity as part of deal limits how high it can go.

"How much [Exelon] might raise its offer is likely limited by the potential need to issue new equity to placate rating agencies and our sense that [Exelon] shareholders remain less than overwhelmed by the acquisition," analysts at Wachovia Capital Markets wrote in a note to clients earlier this week.

Exelon's board is scheduled to meet on June 30 to discuss the hostile bid. Then at NRG Energy's annual meeting on July 21, Exelon is proposing expanding NRG's board and adding its own members as it tries to complete the deal amid the resistance of NRG's current board.

NRG Energy shares were recently down 1.8% to $22.50, while Exelon shares were down 1.5% to $48.21.

-By Mark Peters, Dow Jones Newswires; 212-416-2457; mark.peters@dowjones.com