The real race is on between Romney and Obama

Editor's note: Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" (Times Books) and "Governing America" (Princeton University Press).

(CNN) -- Rick Santorum's announcement that he is suspending his presidential campaign brings the Republican primaries closer to the end. With all the speculation about brokered conventions and last-minute turnarounds, the original predictions appear to be correct. Mitt Romney will almost certainly be the Republican candidate who faces off against President Barack Obama in the fall.

The rebellion against Romney took on many forms since this primary season began. We saw the business conservative (Herman Cain), the ideological and slash-and-burn conservative (Newt Gingrich), the libertarian (Ron Paul), the movement conservatives (Rick Santorum and Michele Bachmann) all try to win the hearts and minds of Republican voters by claiming that they were better suited to this role than Romney.

Julian Zelizer

But in the end Romney wins. We can learn a number of lessons from this outcome. The first is that organization and money do matter. Despite all the interest in surprise victories by his opponents in places like Iowa and South Carolina, the fact that Romney was able to put together such an extensive organizational apparatus early in this campaign, and to raise huge amounts of money to conduct his advertising, made him extraordinarily difficult to beat in several key states. Retail politics could only take his opponents so far.

The second lesson is that Romney is more skilled as a politician than he is often given credit for and his opponents were never as strong as they were made to appear by the media. Romney proved that he had some of the survival skills needed to last through the turbulent ups and downs of the primary process. He was successful in the debates and responded well to the inevitable gaffes and misstatements that were made over these months.

Santorum to 'help' Romney

His insistence on sticking to economic issues, with some diversions to counteract Santorum, reflected an awareness of his own strengths and a resistance to remaking himself in the middle of this contest. He is disciplined on the campaign trail.

Santorum: The 'race for us is over'

Nor were his opponents very strong. Gingrich was a flawed candidate from the start, loaded with baggage from his political career and a well-known tendency to speak what is on his mind regardless of the political costs.

Santorum out, clearing path for Romney

Santorum, too, brought to the campaign his long record in Washington and a weak organizational infrastructure that did not serve him well. Others, like Herman Cain, quickly burned out when under the national spotlight.

Focus shifts to Romney-Obama showdown

Finally, we learned that the anger about President Obama and the frustration over the current course of the economy is greater than any internal debates that exist in the GOP.

While Santorum often did excite many of voters in the primaries and caucuses, he never could close the deal in terms of convincing them that he had a better chance to defeat Obama than did Romney. When faced with a choice between four more years of Obama or a middle-of-the-road Romney, it seems that Republicans prefer Romney.

So now the real race is on.

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The opinions expressed in this commentary are solely those of Julian Zelizer.