O'Hare and Union Station are connected via the CTA's Blue Line and the trip takes 46 minutes and costs $1.80. Get off at the Clinton Station; short walk over to Union Station.

Realistically, given the time it takes to negotiate through O'Hare and claim bags etc., you should probably allow at least 2 hours to make the connection.

There are only 2 daily trains to Champaign; both are all reserved. This means you are supposed to have a ticket for a specific date and train.

The cheapest tickets (save 5% by booking at Amtrak.com for purchases through 12/13; even more savings (another 10%) as a AAA member) are typically "nonrefundable" these days, but can be exchanged for full dollar value for some future Amtrak trip if not used for the intended train; no need to cancel the reservation in advance.

I would probably get a ticket for the train that I think I can get; then if I miss it I would take a chance at having it exchanged at Union Station for the other train. There should be enough space except on the busiest of holiday travel days.