Mitt Romney greets diners at a restaurant during a campaign stop in Rosemont, Ill., on Friday. (JIM YOUNG/REUTERS)

Romney  told a family seated in a booth here at the Pancakes  Eggcetera Breakfast and Lunch spot on Friday that after his trip to Puerto Rico later in the day, he would return to Illinois this  weekend.

The former Massachusetts governor and GOP frontrunner had been scheduled to campaign again in the state on Monday.

Polls show Romney and former senator Santorum locked in a tight race in the state.

Romney's quick stop at this diner in a Chicago suburb not far from O'Hare Airport came as he prepared for his trip to Puerto Rico, where he is favored to win the territorial caucuses on Sunday.

As a toy train made its way around the packed restaurant on a track overhead, Romney spoke to supporters for 10 minutes, focusing squarely on the economy and President Obama.

He began by poking fun at the Obama campaign's release on Wednesday of a 17-minute documentary, directed by Davis Guggenheim, which he said was "obviously an infomercial."

"Now, I found it interesting this morning to hear that the Guggenheim so-called documentary about President Obama is now on the Internet. ... Mr. Guggenheim said that he could find nothing wrong to say about the president -- nothing negative to say in this documentary," Romney said.

He suggested that Guggenheim talk to some of those who are "struggling because gas prices have gone through the roof."

He also contrasted himself with the rest of the GOP field as the candidate best-equipped to take on Obama on the issue.

"Now, to beat Barack Obama, we're going to have to be convincing on the concern people talk about day in and day out, the number-one issue in America: the strength of our economy," Romney said.

On that matter, he said, "this president's a lightweight."

"Now, it's not because he's not smart," Romney said. "It's because he's never worked in the free economy. He's never had a job in the free economy. It's hard to create a job if you've never had one."

He highlighted his 25 years in the private sector and his tenure as Massachusetts governor and as chief executive of the Salt Lake Olympics committee. Then Romney took a veiled shot at Santorum's years in Washington as a member of the U.S. House and then the Senate from Pennsylvania.

"I understand how the economy works -- not because I debated it in Congress, but because I've worked in the real economy," he said. "We're not going to be successful in replacing an economic lightweight if we nominate an economic lightweight, and I am an economic heavyweight." Santorum was also in Illinois on Thursday.

As the candidate worked the breakfast crowd, the enthusiasm for Romney was clear -- and Romney’s characteristic wry sense of humor was on display.

"Good morning. Table for six, please," he joked to the hostess as the event got underway.

He held up a baby as throngs of reporters and videographers climbed on top of the booths to get a view.

"Are you going to kiss the baby?" one woman asked.

"Oh, you've always got to kiss the baby. ... Especially when the diapers are dry," Romney responded with a laugh.

He also made a crack at Chicago’s reputation for ruthless politics.

"I need you to vote. I need you all to vote,” Romney said. “And, by the way, you're allowed to vote multiple times -- by getting a friend to go with you. I know this is Chicago -- I had to clarify."

Romney was accompanied at the event by several local officials and predicted that with their support, he would emerge the victor on Tuesday.

But his stepped-up campaign schedule suggests that he believes he's in for a tougher race than he might have been anticipated.

About a dozen people, including Rep. Jan Schakowsky (D-Ill.), held signs outside the diner on Friday criticizing Romney for his position on Planned Parenthood and contraception coverage.