SPRINGFIELD, Ill. (Reuters) - A standoff between Democrats who control the Illinois Legislature and the state's Republican governor may extend well into the summer after no deals emerged ahead of the end of the spring legislative session on Sunday.

Senate President John Cullerton, a Chicago Democrat, said that it was unlikely Democrats and Republican Governor Bruce Rauner will reach any deals on the budget and other matters before the key session wraps up later on Sunday.

That would leave Illinois with a new state budget that has a revenue shortfall of at least $3 billion.

Cullerton later ordered the Senate back into session on June 9 while House Speaker Michael Madigan, also a Chicago Democrat, ordered his chamber back into session next Thursday. Lawmakers will be in continuous session with as little as 48 hours notice throughout the summer.

Cullerton told reporters that instead of working with Democrats on the spending plan, Rauner has been working on a multimillion-dollar advertising blitz aimed against Democratic lawmakers.

Representatives for Rauner did not immediately respond to a request for comment.

Democrats who control the House and Senate opted for their own $36.3 billion spending plan for the fiscal year that begins July 1 instead of a $32 billion budget proposed by Rauner in February. House Speaker Michael Madigan acknowledged last Monday that the Democrats' budget has insufficient revenue.

Cullerton said the governor can either cut spending in the budget or agree on new revenue to plug the gap.

Rauner said on Friday that he will not sign an unbalanced budget, but he also said he would not force lawmakers into an expensive special session this summer.

The governor has said he has whittled down his so-called turnaround agenda of reforms he wants the legislature to pass.

"I want to work with him on his turnaround agenda but much of the agenda in its current form, its theme is middle-class workers lose and corporate CEOs win," Cullerton said.

He added that deploying an ad campaign would be damaging to the prospects of compromise.

Rauner aimed his reforms at issues such as workers' compensation, right-to-work zones and local property taxes.

(Reporting by Karl Plume, additional reporting by Karen Pierog in Chicago; Editing by Eric Walsh)