The former Massachusetts governor clinched his victory with a 12-point lead over Rick Santorum, the runner-up, in Obama's home state. Romney won most of the 54 delegates up for grabs Tuesday and solidified his position as the front-runner in this year's unusually long race for the Republican nomination.
With 99 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 (8 percent).
Not all of the delegates had been officially awarded to the candidates as of late Tuesday, but the Associated Press reported that 40 delegates will support Romney and represent him in August at the Republican National Convention in Tampa, Fla. Additional delegates -- Illinois has a total of 69 -- will be chosen at a later time by party leaders.
Romney's delegate tally will push him even further ahead of Santorum -- before Tuesday's primary, the front-runner had 522 delegates to Santorum's 252. A total of 1,144 delegates are needed to secure the Republican nomination and face Obama.
When Romney and his wife, Ann, took the stage in Schaumburg, Ill., on Tuesday night, only 45 percent of precincts had reported their results, but media outlets had already projected him the winner. He spent most of his speech hammering "Professor" Obama, who Romney claims couldn't have learned about the economy as a University of Chicago law professor.
"It's time to say these words, this word -- 'enough.' We've had enough," Romney said to cheering supporters.
The Illinois win, as well as his overwhelming victory in Puerto Rico over the weekend, was what Romney needed to pad his lead in a race in which no Republican has emerged as the undisputed standard-bearer.
Polls leading up to Tuesday's vote had Romney in the lead by an average winning margin of 10 points, according to Real Clear Politics. Still, Santorum had been hoping for an upset like the ones he scored last week in Alabama and Mississippi, propelling him into a two-man race with Romney.
Although Pennsylvania's primary in April looks promising for native son Santorum, other upcoming contests in Washington, D.C., New York, Connecticut and elsewhere appear Romney's to win.
Tuesday marked the beginning of the end for Santorum. Below are three big reasons he lost Illinois, and why he'll probably lose the nomination.
Exit poll data collected by CNN indicated that Illinois had significantly different demographics than in Alabama and Mississippi, where Santorum won and Romney came in third place. The Prairie State lacked the conservative, evangelical and religious voters that helped him sweep the Deep South, giving Romney a competitive edge.
Forty-two percent of Illinois primary voters identified themselves as evangelical or born-again Christians, while 31 percent said they were "very conservative." In both Mississippi and Alabama, about 80 percent said they were evangelical Christians; in Mississippi, 42 percent considered themselves "very conservative," compared with 36 percent of Alabama voters.
Santorum's popularity among those more conservative voters, as well as his favor among voters from rural parts of Illinois, helped him win many counties downstate. Unfortunately for him, Romney won over Chicago and surrounding counties, which are much more densely populated and politically moderate.
In his speech in Gettysburg, Pa., Santorum tried to play up the exit poll numbers and paint himself as the real "conservative" in the race. "We won the areas that the Republicans and the conservatives populate," he said.
Santorum is a favorite among many on the far right, but his inability to win over moderates is a debilitating weakness, one that would become even more pronounced in the general election. And he isn't the universal favorite among all Tea Party movement supporters and evangelical Christians. For example, he lost badly to Romney on Tuesday in Illinois's Peoria County, 46.6 percent to 37.5 percent, which has a strong Tea Party presence that the Washington Post described as representative of middle America.
Despite pleas by Republican candidates for Illinois voters to show up Tuesday, the primary had very low turnout -- a likely sign of voters' lack of enthusiasm.
Throughout Chicago, voter participation was "extremely low" Jim Allen, a spokesman for the city's election board, told the Chicago Sun-Times. Some of the lowest-turnout areas were Republican strongholds.
Only one in five registered voters cast ballots in Sangamon County, a largely conservative area in central Illinois, according to the Associated Press. The same ratio of voters in the Republican-leaning suburbs of Cook County made their ways to the polls.
Santorum realized that turnout was critical to any success he might hope for. In Illinois last week, he told a group of supporters that "turnout is everything."
But only 47 percent of voters expressed strong suppor for their candidate, while 40 percent said they "have reservations" and 11 percent said they disliked the other candidates.
More than three months into the Republican campaign, no other Republican candidate -- including Santorum -- has been able to drum up enough excitement to dislodge Romney from his perch as inevitable nominee.
The most telling bit of data from Tuesday's exit polls: the degree to which voters considered the ability to beat Obama the most important quality for the Republican nominee. A total of 36 percent said electability was a candidate's most important attribute, while 23 percent cited "strong moral character" as No. 1. Another 19 percent said being a "true conservative" was most important and 18 percent looked for "the right experience" in their candidate.
Electability is a quality that voters across the United States value in a candidate, regardless of how conservative a state is. In Mississippi, 39 percent of Republican voters said defeating Obama was the most important factor in a Republican presidential nominee, compared to 20 percent who selected "true conservative" and "strong moral character." In Alabama, 35 percent said electability was the most important.
Not surprisingly, Romney dominated among voters who thought a candidate's chance of taking down Obama was his most important trait. In Illinois, 74 percent of voters who had that priority in mind voted for Romney.
Romney's victory in Illinois is a major blow to the Santorum campaign because it proves an inevitable truth about the 2012 presidential election: At this point, Republicans just want someone they think can beat Obama.
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