Former Speaker Dennis Hastert (R-Ill.) is expected to announce that he will resign his seat in Congress effective later this year, eventually setting up a special election to succeed him, knowledgeable GOP sources said late Wednesday.

A timeline for the official announcement remains unclear, after Hastert scrapped initial plans to make the announcement today.

Rumblings have persisted for months that Hastert, who announced this summer that he would not seek re-election in 2008, was unlikely to complete his current term.

This week Hastert met with House Minority Leader John Boehner (R-Ohio) and was making calls to tell people of his decision on Wednesday morning.

One key issue for Republicans is the timing of a special election, which will be called by Illinois Gov. Rod Blagojevich (D), and whether it might be held in conjunction with the state’s Feb. 5 presidential and Congressional primaries.

Several candidates already are vying to succeed Hastert. The leading Republicans are state Sen. Chris Lauzen, dairy magnate Jim Oberweis and Geneva Mayor Kevin Burns. The leading Democrat is wealthy scientist Bill Foster.

The GOP would be slightly favored to hold the exurban Chicago seat in a regular election, but a special election could be problematic for the GOP because the Democratic Congressional Campaign Committee has far more money than the National Republican Congressional Committee.

Hastert’s earlier departure has significant political implications for Republican politics in the Land of Lincoln. First, it set up a possible double Feb. 5 primary to temporarily fill the seat until January 2009, and set the matchup for Election Day 2008.

Having both primaries on the same day may stoke fears of last year’s head-scratcher in former House Majority Leader Tom DeLay’s (R-Texas) Houston-area district. After DeLay’s early departure, Shelley Sekula Gibbs (R) won a special election to replace DeLay for the remaining months of his term. Democrat Nick Lampson won the general election to replace DeLay.

According to state law, Blagojevich has five days from Hastert’s last day in office to schedule the special election, which must take place within 115 days.

Although the law is clear regarding when special elections must be held state law is vague regarding whether a special primary precede it.