Poll: Romney lead over Santorum in Illinois is widening (VIDEO)

A poll released Monday gives Mitt Romney a double-digit lead over rival Rick Santorum in the battleground state of Illinois headed into Tuesday's primary.

The American Research Group survey showed Romney with support from 44 percent of likely GOP primary voters, compared with 30 percent for Santorum, CNN reported.

The 14-point margin is a marked increase for Romney, who stood only four points ahead of his rival in a Chicago Tribune/WGN poll taken at the beginning of March, according to CNN.

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Newt Gingrich and Ron Paul came in third and fourth, respectively, and lag far behind the two GOP leaders, USA Today reported.

Romney, who won the Puerto Rico primary on Sunday, and Santorum were campaigning Monday in Illinois. The state has 54 delegates at stake in its Tuesday primary, out of a total of 69.

In an appearance Monday morning on CBS' The Early Show, Santorum said he felt good about upcoming contests.

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He contends the GOP nomination will be decided at the party's national convention in Tampa, Fla., this summer because Romney will not secure the required 1,144 delegates by the time primaries are over in late June.

"The convention will nominate a conservative. The convention will not nominate a moderate," Santorum told CBS. "We are going to continue to work hard to make sure a conservative is the nominee of the party."

Romney's campaign, meanwhile, released a new Web video using Santorum's words from 2008 against him. In that year's presidential contest, Santorum urged voters to back Romney "if you want a conservative as the nominee of this party," according to USA Today.