With 98 percent of results in, Romney received 46.7 percent of votes in Illinois. Santorum was second with 35 percent, followed by Texas congressman Ron Paul (9.3 percent) and former House of Representatives speaker Newt Gingrich (7.9 percent).
It's time to say these words, this word -- 'enough,' Romney told supporters at a victory rally in Schaumburg, Ill. We know our future is brighter than these troubled times. We deserve a president who believes in us. And I believe in the American people.
Santorum, speaking in Gettysburg, Pa., said he had phoned Romney with congratulations. The former U.S. senator from Pennsylvania said he was happy to have won some conservative parts of Illinois.
The Republicans' primary race now moves to Louisiana, where voters go to the polls Saturday.
Romney added to his already-insurmountable lead in delegates, well on his way to attaining the 1,144 needed to become his party's challenger to Obama in November.
Before Tuesday's voting began, a Public Policy Polling survey had Mitt Romney 15 points ahead of Rick Santorum, 45 percent to 30 percent, respectively, while a Fox Chicago/WAA poll showed Romney leading with 37 percent of the vote to Santorum's 31 percent.
Santorum, however, had been a speed bump on Romney's road to the nomination ever since the ex-senator's first multi-state sweep in early February, when he won Colorado, Minnesota and Missouri in one night.
Fifty-four of Illinois' 69 delegates were at stake in Tuesday's primary. For live coverage of the results from the Prairie State, including information about exit polls, the percentage of the number of votes tallied, and other information, click on the links below to find comprehensive coverage of the 2012 Illinois Republican primary:
9:50 p.m.: Fifty percent of the results are now in for the Illinois Republican primary. Mitt Romney has won a sweeping victory in the state, with 50 percent of the vote to Rick Santorum's 33.2 percent.
9:40 p.m.: Forty-six percent of the vote is in. Romney has 50.2 percent of the vote to Santorum's 32.3 percent. Ron Paul is in third with 9.2 percent, with Newt Gingrich in fourth with 7.5 percent of the vote.
9:25 p.m.: With 38.7 percent of the vote in, Mitt Romney has been declared the winner of the Illinois Republican primary with 51.4 percent of the vote to Rick Santorum's 31.2 percent.
9:20 p.m.: In the final hours of his Illinois campaign, Santorum made a serious gaffe when it came to discussing economic policy, one that may be indicative of his failure to reach voters in the less socially conservative state.
On Tuesday, Santorum appeared to shrug off the importance of the economy in the race, saying, I don't care what the unemployment rate's going to be.
It doesn't matter, he added. My campaign doesn't hinge on unemployment rates and growth rates. There's something more foundational that's going on here.
Romney's campaign, after hearing of the soundbite, informed their candidate, who quickly launched an attack against Santorum.
One of the people who is running also for the Republican nomination today said that he doesn't care about the unemployment rate, that does bother me, Romney said at an event in Peoria, Ill. I do care about the unemployment rate. It does bother me.
9:15 p.m.: Romney has been declared the winner of the 2012 Illinois Republican primary by numerous media outlets, with 32 percent of the vote in so far. He currently has 54 percent of the vote. Santorum is in second with 29 percent of the vote. Ron Paul remains in third with 9 percent, with Newt Gingrich in fourth with 7 percent of the vote.
9:10 p.m.: Exit polling from ABC in Illinois continues to support early results from Illinois. Six in 10 Illinois voters believe that Romney is the GOP candidate with the best chance of beating President Obama in the fall. Romney also leads Santorum, though narrowly, as the candidate who best understands the problems of average Americans.
Although four in 10 Illinois voters also said that Romney was not conservative enough, a common criticism of the moderate-leaning candidate, but many also say that Romney, like Santorum, is about right when it comes to the issues voters considered most important in the coming election, a significant difference from perception of Romney in Alabama, Mississippi, Georgia and Tennessee.
9:05 p.m.: According to Politico's Alexander Burns, Romney outspent rival Rick Santorum in Illinois by a ratio of 7 to 1. Romney spent $1.1 million on ads in Illinois, while the super PAC supporting him provided an additional $2.6 million in funds. Rick Santorum, in contrast, spent only $219,961 while the super PAC behind him put in $312,150.
9:00 p.m.: Twenty-five percent of the vote reported from Illinois, Mitt Romney continues to have a significant and possibly unsurpassable lead over Rick Santorum, with 56 percent of the vote to Rick Santorum's 27.3 percent.
8:55 p.m.: Only Mitt Romney, of the four candidates left in the Republican race, is in Illinois tonight for the GOP primary. The former Massachusetts governor is camped out in Schaumburg, Ill., for a results watching party, and plans to bypass Louisiana and head for Maryland on Wednesday.
Rick Santorum, meanwhile, is spending the night in Gettysburg, Pa., before heading to Louisiana on Wednesday, where Newt Gingrich has already placed himself in anticipation of its own primary contest on Saturday.
8:50 p.m.: With 15 percent of the vote in, Mitt Romney continues to hold a strong lead over Rick Santorum, with 54.5 percent of the vote to Santorum's 27.5 percent. Ron Paul and Newt Gingrich hold third and fourth place with 9.5 and 6.7 percent of the vote, respectively.
8:45 p.m.: Exit polls also indicate only 8 percent of Illinois primary voters were between ages 18 and 24, a key voting bloc for independents. About 50 percent of voters, according to CNN, were between 45 and 64.
8:43 p.m.: According to early exit polls by the Huffington Post, the vast majority of the Illinois voters who took part in Tuesday's Republican primary were white, about 98 percent. Census data indicates Illinois's popultion is 71 percent white and 14.5 percent black.
8:40 p.m.: GOP voters in Illinois appear relatively unenthusiastic about the candidates up for the Republican nomination, no matter who they're supporting. About half those polled say they strongly favor their candidate, lower than in states like Alabama and Iowa, with almost all of the rest saying they cast their vote with reservations.
Despite this, however, nearly 70 percent of GOP voters also said they were committed to their candidate no matter how long the race lasts, even if the contest drags out for many months to come.
8:35 p.m.: With just five percent of the vote in, Mitt Romney leads in Illinois with 54 percent of the vote to Rick Santorum's 26.6 percent. Ron Paul is in third with 12.1 percent, and Newt Gingrich trails with 6.3 percent of the vote.
8:25 p.m.: Newt Gingrich and Ron Paul may technically be contenders in the Illinois Republican primary, but Mitt Romney and Rick Santorum exhausted all their ammunition on each other in the days before the Tuesday contest. On Monday, after reports that Romney was calling him an economic lightweight, Santorum shot back.
If I am a lightweight, I agree he is a heavyweight, he is a big-government heavyweight, Sanotrum responded. That's what his record was.
8:20 p.m.: In another bad stroke of luck for Rick Santorum, Politico Live reports that the GOP contest in Illinois is expected to have the lowest turnout in presidential primary history. Turnout is everything, Santorum said at a recent campaign stop, words that may turn prophetic as the night goes on.
NBC News, however, despite Romney's initial lead in the polls, is still calling the race in Illinois too close to call.
8:15 p.m.: Early exit polls indicate that many of the strengths of Rick Santorum's campaign that aided him in the South--evangelical voters, social conservatism and a lower income--are missing in Illinois, giving Mitt Romney an edge in the competition.
In both Mississippi and Alabama, whose primaries Santorum won last week, 8 in 10 said they were evangelical Christians. Forty-two percent of Mississippians considered themselves very conservative, compared with 36 percent of Alabama voters. And just 23 percent and 26 percent of Alabama and Mississippi voters, respectively, had a total family income of over $100,000.
According to MSNBC's First Read, however, only four in 10 Republican primary voters considered themselves evangelicals. Only 31 percent felt they were very conservative. And when it came to income, 37 percent of Illinois voters could boast a total family income of $100,000 or more.
8:06 p.m.: The Illinois primary race may experience delayed results from the polls due to a problem with size of voters' ballot. According to the Washington Post, about a quarter of the state's counties discovered that they were stuck with ballots that were too wide to fit in the optical scanning machines this afternoon. Many of those ballots will have to be trimmed, and many more will have to be hand-counted after the polls close.
8:03 p.m.: So what about the delegates actually up for grabs in Tuesday night's primary? Illinois, like many other states, awards its delegates proportionally. Each of the state's 18 congressional districts elects two to four delegates to the Republican National Convention (in August this year), with the delegates allotted to the winner of each district.
8:00 p.m.: Only 54 of Illinois' 69 delegates are actually up for grabs in Tuesday night's primary, with the remaining 15 designated as RNC delegates. But Illinois doesn't hold a normal primary. Instead, it runs what is called a loophole primary for presidential elections, meaning the contest is actually split between a presidential preference (or beauty contest) vote and a separate primary that apportions delegates among the candidates.
What does this mean for Illinois voters? Essentially, that voters have the opportunity to indicate a preferred candidate from among the list of possible options, but will cast their real vote for the delegate-awarding primary in a separate column. The results of the beauty contest poll are supposed to be purely advisory, with the state's National Convention delegates throwing their support behind the winner. The contest, however, is completely non-binding.