AP Photo/Getty Images

CHICAGO - Basking in the glow of his commanding win in Illinois on Tuesday night, Mitt Romney delivered his victory speech in Schaumburg, Ill., but it may be another  state 40 miles north  that once and for all seals the deal for the GOP front-runner: Wisconsin.

While Romney focused his victory speech on a general election showdown with President Obama and his top strategist Eric Fehrnstrom called on other candidates to "step aside," the Republican primary seems destined to continue at least through April 3, when Wisconsin, along with Maryland and the District of Columbia, heads to the polls. It is there that Romney will have a real chance to end what has been a long, drawn-out primary.

With a 300 delegate lead over his closest rival, Rick Santorum - 563 to 263 - Romney, as evidenced by his speech in Schaumburg, is eager to focus his attacks on Obama rather than his fellow Republican hopefuls. Romney's resounding triumph in Illinois - he took 47 percent of the vote compared with 35 percent for Santorum, a difference of more than 100,000 votes - gives the former Massachusetts governor a compelling argument for Santorum, Newt Gingrich and Ron Paul to exit the race. But not one of the three has shown any inclination to do so. Gingrich and Paul, at this point, have become irrelevant in the primary, but Santorum still holds out faint hope of making a comeback. If he is to pull that off,  he has to score an unexpected victory in the very near future.

Santorum, it seems, will likely win in Louisiana on Saturday, but that will do little to change the momentum of the race. Come April 3, the former Pennsylvania senator will be hard-pressed to find a lot of support in Maryland and is not even on the ballot in D.C. After that, the next round of primaries does not take place until April 24, when New York, Connecticut, Delaware, Rhode Island and Pennsylvania all vote. Only Pennsylvania is  remotely friendly territory for Santorum, but even there the voters  kicked him out of the Senate after one term. It's not exactly a great home-field advantage.

That all makes April 3 in Wisconsin the last best chance for Santorum to get back into the race. Win the Badger State and Santorum will be back in the conversation, at least until April 24. Lose, and he will merely be an afterthought going forward, a dead man walking. Santorum will waste little time in making his case to Wisconsin voters. He heads there Saturday to address a conference in Milwaukee. Romney is also expected to visit the state later this month, while his wife, Ann, will campaign in Madison and Milwaukee on Thursday.

If past is prologue, Santorum will know that the road ahead is an uphill climb. In the past three Midwestern states to vote - Illinois, Ohio and Michigan - Santorum has suffered crucial defeats. In each state, Romney had better organization and far more money, a lethal combination for his rivals. In Illinois, for instance, Santorum failed to qualify for the ballot in four of the state's 18 congressional districts and was outspent by an eight to one  margin statewide and a 21 to one margin in the Chicago area. Making matters worse, Santorum did himself no favors by declaring that he did not care about the unemployment rate - this in a state where the jobless rate is a full point higher than the national average. What had once appeared to be a close race in Illinois ended up being anything but.

Romney and his allies have already started to flex their financial muscle in Wisconsin. His super PAC - Restore Our Future - has bought around $1.3 million worth of airtime in the state, with ads on the air dating back to March 15. The Santorum campaign, by contrast, has  shelled out only $54,000 in Wisconsin to date, and his super PAC - the Red, White and Blue Fund - has yet to spend a dime there.

That does not bode well for Santorum's chances in a state that has 42 delegates on offer and the potential to snuff out any hope he has of pulling off a shocking upset. Another factor working against him is that Romney is viewed as the favorite in Waukesha County, a wealthy, educated area just to the west of Milwaukee. Romney so far has used his strength in the most urban areas of Illinois, Michigan and Ohio to catapult him to statewide victories. In Illinois, it was the Chicago area. In Michigan, around Detroit. And in Ohio, Cleveland, Cincinnati and Columbus. In Wisconsin next month, Romney will hope to replicate that magic in Milwaukee and Madison.

Do not be surprised if he succeeds. And if Romney does take the Badger State - as well as D.C. and Maryland that same day - then it will be fair to declare the GOP race over once and for all, even if Santorum, Gingrich  and Paul continue to argue otherwise.

Matthew Jaffe is covering the 2012 campaign for ABC News and Univision.