Former Speaker J. Dennis Hastert, R-Ill., reduced to minority party backbencher by the Democratic triumph in the 2006 elections, will resign from the House before the end of this year.

Hastert’s office said “he plans to formally resign from the House before the end of the year.” Hastert said in an interview Thursday he did not plan to announce a specific date for his resignation now. “I will leave when I finish my work,” he said.

The longest-serving Republican Speaker in history, Hastert had previously announced that he would retire at the end of the 110th Congress.

Ending his 11-term political career early would force a special election in Hastert’s 14th District in north-central Illinois, but it also would give his successor a small advantage in seniority — provided that the victor wins again in November 2008.

Hastert’s early retirement had been rumored in August, before he announced that he would not run again.

Democratic Gov. Rod R. Blagojevich has wide discretion in scheduling a special election to fill the upcoming vacancy in Hastert’s 14th District. But Hastert is expected to time his resignation in a way that prevents the special election from coinciding with the Feb. 5 primary elections.

A large Democratic turnout is likely that day, spawned by the presidential primary campaign of Illinois Sen. Barack Obama. That could help the Democrats if the House special election were held at the same time.

Four Republicans and four Democrats have filed paperwork to compete in the regular primary election on Feb. 5, and the major candidates have long prepared for the likelihood of a special election.