WASHINGTON (AP) — Former Massachusetts Gov. Mitt Romney added to his lead in the race for delegates Tuesday by winning the Republican presidential primary in Illinois.

Romney won at least 41 delegates in Illinois and former Pennsylvania Sen. Rick Santorum won at least 10. The two rivals were battling for the last three delegates at stake in the primary, but the results were too close to call on election night.

Former House Speaker Newt Gingrich and Texas Rep. Ron Paul were shut out.

Illinois has a unique system of awarding delegates to candidates. The 54 delegates at stake Tuesday were not determined by the statewide vote. Instead, individual delegates were listed on the ballot in each of the state's 18 congressional districts. They were identified by the candidate they support.

Santorum was at a disadvantage because only 44 of his delegates qualified for the ballot.

Romney leads the overall race for delegates with 563, including endorsements from Republican National Committee members who automatically attend the convention and can support any candidate they choose. Santorum has 263 delegates, Gingrich has 135 and Paul has 50.

It will take 1,144 delegates to win the Republican nomination to take on President Barack Obama in the fall.

So far, Romney has won 54 percent of the delegates at stake in primaries and caucuses. That puts him on pace to clinch the nomination in June, but it doesn't leave much room for error.

Santorum has won 27 percent of the primary and caucus delegates at stake so far. He would need to win 74 percent of the remaining delegates to clinch the nomination before the party's national convention in August.