WASHINGTON – Former House Speaker J. Dennis Hastert will resign before his term ends 15 months from now, closing a 21-year career in Congress, Republican aides said Thursday.

The Illinois Republican announced in August he would not seek re-election in 2008, but it was unclear whether he would finish his current term, which runs until January 2009. Republican aides, speaking on condition of anonymity because Hastert had not publicly announced his new plans, said the lawmaker now has told colleagues he intends to leave office late this year or early next year.

Hastert’s resignation would mean a special election, probably in early 2008, in his Chicago-area district that includes Aurora and Elgin. Republicans would like to secure the seat ahead of the November 2008 election, which some fear will draw large numbers of Democratic voters unhappy with President Bush and the Iraq war.

Speaking to two reporters Thursday in Washington, Hastert would not confirm or deny the resignation reports.

Hastert, 65, was the longest-serving Republican speaker in history. He took the post in January 1999 after two better-known party members were forced aside by scandals and intraparty rivalries.

When Democrats regained control of the House after the 2006 elections, Hastert lost the speakership. His retirement plans did not surprise colleagues.