SPRINGFIELD, Ill. — Emil Jones, who helped Senator Obama reach the U.S. Senate and has been Governor Blagojevich’s staunchest ally, said yesterday he will not seek re-election in November and plans to retire at the end of his term.

The 72-year-old president of the Illinois Senate thanked colleagues and constituents in a statement released by his office, but the 35-year veteran of the Legislature didn’t say why he was retiring.

Mr. Jones told Crain’s Chicago Business he wanted to spend more time with his family. He declined to comment when reached by the Associated Press.

Mr. Jones’s decision leaves a power vacuum in a state marked by discord among its Democratic leaders and means the unpopular Mr. Blagojevich is losing one of his most important political allies. His decision already has set off a scramble among Senate Democrats to replace him.

It also means the state is losing one of its most powerful black politicians and a leading advocate for education reform.

Mr. Jones had a big hand in Mr. Obama’s 2004 U.S. Senate win by introducing the then-little-known state senator to the right people and letting him handle some important legislation to help raise his political profile.

Mr. Jones also is well known for sticking by Mr. Blagojevich. That includes siding with the Democratic governor in his most recent battle with another fellow Democrat, the House Speaker Michael Madigan, over a stalled statewide construction program. Mr. Blagojevich’s office did not immediately comment yesterday.