Illinois was traditionally a bellwether state, picking the losing presidential candidate just once between 1920 and 2000. But in 2000 the state chose Al Gore, and it has been moving further into the Democrat column ever since.
Although the state's House delegation remains evenly split, in 2002 Democrats won control of the governor's mansion, and since 2004, when Barack Obama won his senate race, the party has controlled both Illinois Senate seats.
The presence of Mr Obama on the presidential ballot will only serve to make the state even safer for the Democrats in November.
Population: 12,831,970 (ranked 5 among states)
Governor: Patrick Quinn (D)
Electoral College votes: 21
The Democratic Party dominated Illinois's biggest city, Chicago, for much of the 20th Century. Party bosses had influence at all levels of society, controlling appointments and patronage and engineering the vote.
Among these bosses was the legendary Chicago Mayor Richard Daley, who had a dramatic effect on two presidential contests.
In 1960, he was instrumental in drumming up support for John F Kennedy, and in 1968 his police clashed with anti-war demonstrators at the Democratic Convention, damaging the city's political psyche and the party's image.
House of Representatives:
10 Democrat, 9 Republican
Senate: 2 Democrat
The Democratic Convention returned in 1996 and the political machine has largely gone. But the current, highly popular Chicago mayor is Richard Daley's son.
Illinois is an industrial leader. By the end of the 19th Century, the state was established as a centre of urban industry. Chicago grew as the rail network spread and soon became the leader in the US meatpacking industry.
It is still an important manufacturing centre and it has the largest service and white-collar sector in the interior of the US. But now the trading of commodities, rather than of meat, dominates the city's economy.
2004: Bush 44%, Kerry 55%
2000: Bush 43%, Gore 55%
1996: Clinton 49%, Dole 37%
Illinois's economy has attracted millions of immigrants to Chicago over the decades.
It is the third largest city in the US, with the world's second busiest airport and a university that employs seven Nobel laureates.