Washington, Mar 20 (IANS): Two leading Republican presidential hopefuls have launched a final campaign blitz across Illinois, President Barack Obama's home state, ahead of their showdown in a key primary Tuesday.

While Mitt Romney is hoping that the primary would cement his front-running status, his closest rival Rick Santorum is eyeing a conservative-backed surge in the Republican presidential race.

Tuesday's Illinois primary offers 54 delegates and follows a strong Romney victory in Puerto Rico, where the former Massachusetts governor picked up all 20 delegates at stake on Sunday by winning more than 80 percent of the vote.

CNN's latest delegate estimates show Romney with 519 delegates to former senator Santorum's 239. Former House Speaker Newt Gingrich has 138 delegates and House member Ron Paul, the libertarian champion, has 69.

To secure the nomination, 1,144 delegates are needed. In the Illinois contest, delegates will be awarded proportionately.

Despite his growing lead in the delegate count, Romney still faces questions about his ability to win support from the Republican conservative base and needs a victory in Illinois to answer doubters, CNN said.

An American Research Group poll released Monday indicated Romney had a solid lead over Santorum.

According to the survey conducted over the weekend, 44 percent of likely Republican primary voters favoured Romney compared with 30 percent for Santorum, 13 percent for Gingrich and 8 percent for Paul. The sampling error was plus or minus 4 percent.

Santorum already has won contests in neighbouring Minnesota, Iowa and Missouri. Capturing Illinois would continue his momentum in the region.

Romney kicked off his Monday campaigning in Springfield, saying that he has the economic gravitas to bring growth and job creation, unlike an "economic lightweight" like Obama.