Quick answer: no portability, but Illinois offers its own, independent QTIP election that can be even more powerful. Get a calibrated estimate in the Illinois estate tax calculator.
Portability, ruled out explicitly
The Illinois-only QTIP, and how powerful it can be
For deaths on or after January 1, 2009, an estate may make an Illinois QTIP election independent of any federal QTIP election, deferring Illinois tax on qualifying property passed to a surviving spouse. The Attorney General's own published example shows exactly how far this can reach: a $13,610,000 estate, with a surviving spouse and an Illinois QTIP election of $9,610,000, owes $0 Illinois estate tax — the election brings the taxable amount down to precisely the $4,000,000 exclusion.
It extends to civil unions, unlike federal law
Parties to a civil union recognized under the Illinois Religious Freedom Protection and Civil Union Act are treated the same as spouses for the Illinois marital deduction and QTIP election, even though civil unions aren't recognized for federal estate tax purposes at all. A civil union partner making this election must still file a full set of returns, including a pro forma federal Form 706 completed as if federal law allowed the deduction.
The election must be made correctly on the return
The Illinois QTIP is elected on a timely filed Form 700 by checking the election box, entering the dollar amount, and providing the surviving spouse's Social Security number, along with a list of the specific QTIP property — including the percentage included when the property sits inside a trust.
A local probate attorney can review your estate — many offer a free consultation.
Because Illinois's underlying tax computation is itself an interrelated calculation (see the threshold and rates guide), the exact benefit of any QTIP election is best confirmed using the Attorney General's own calculator.