There's a potential bright spot in Illinois, where a special political action committee that supports him is already airing TV ads.

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Romney faces the extended calendar with a smaller financial advantage than the one he enjoyed through the campaign's early months.

He announced Wednesday that he raised $11.5 million in February, but rival Santorum has said he brought in more than $9 million.

Romney was able to close significant gaps with Santorum in places like Ohio and Michigan by spending significant amounts of money on TV ads.

Romney also has acknowledged making a series of campaign mistakes and has said he is working to improve as a candidate. In the CNBC interview, he acknowledged that it's impossible to know whether his plan to cut marginal tax rates by 20 percent will add to the federal deficit.

"It can't be scored," Romney said. He said details still need to be worked out with Congress. Romney also shifted his position on the minimum wage this week, telling CNBC that "there's probably not a need to raise the minimum wage."

Earlier this year, Romney said he hadn't changed the position he held when he was governor of Massachusetts, which is that the minimum wage should rise automatically along with inflation.

Romney took a break from campaigning Wednesday and spent the day at home in Massachusetts.