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.

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."

But the Chicago power company is facing growing pressure to raise its offer as shares of NRG Energy trade near the exchange offer. At the same time, 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.

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.

NRG Energy shares were recently up 0.2% to $22.94, while Exelon shares were flat at $48.94.

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