Maryland's Republican voters, long marginalized in the selection of the president, may have contributed to the tipping point in the long and unpredictable GOP nominating process. Former Massachusetts governor Mitt Romney won the Old Line State easily, along with neighboring Washington, D.C. That was no surprise; the relatively affluent, urbanized electorate here has been a strong Romney constituency from the beginning. But the number of delegates Mr. Romney cleared from those contests and from what another win in Wisconsin should mark the beginning of the end of the 2012 primaries and foretell a shift toward the broader debate that will take place before November's general election.

Before today's voting, former House Speaker Newt Gingrich, who failed to attract consistent support even in the Deep South, was already beginning to wind down his campaign. Texas Rep. Ron Paul remains likely to stay in the race, thanks to the passion of his supporters, but he has shown himself unable to expand his base of support. And now it should be clear to Rick Santorum, the most successful of the non-Romneys, that the path for him to secure the nomination, even through a brokered convention, is all but impossible.

Even if Mr. Santorum had pulled off a massive upset in Wisconsin, Mr. Romney was certain to increase his advantage in the delegate count after Tuesday's voting. Based on the results so far, it appears that the night was disastrous to Mr. Santorum's hopes of checking the Romney advance. Mr. Romney swept all of the delegates awarded in Maryalnd and Washington, and appears likely to come close to repeating the feat in Wisconsin. Even before all the results were in Tuesday, New York Times elections statistics guru Nate Silver had pegged Mr. Romney to have a 96 percent chance to clinch the nomination before the GOP convention. At a time when national polls are swinging Mr. Romney's way, Mr. Santorum would have to nearly run the table in the remaining contests to get the 1,144 delegates needed to win the nomination outright, and he would need to significantly outperform his results so far to prevent Mr. Romney from doing so.

And for Mr. Santorum, the calendar is not kind. After the fast pace of voting in the first three months of the year, we now enter a three-week lull before the next Republican contests. That will make it difficult for Mr. Santorum to establish any momentum — and to raise the money necessary to compete. The primaries that come next, on April 24, are unlikely to help. The Pennsylvania primary is that day, and Mr. Santorum leads in all recent polls in his home state, which has 72 delegates. But the other contests that Tuesday are in Connecticut, New York and Rhode Island. Together, they offer 142 delegates, and Mr. Romney is heavily favored in all three.

That means it will soon be time to begin the debate over the fall general election in earnest. It is unlikely that Mr. Romney will be able to reset his campaign entirely, as one of his top aides recently suggested by comparing the general election contest to an Etch-a-Sketch. He will not be able to disavow his efforts to sell himself to a Republican primary audience as "severely conservative." But he will be forced to address himself to a broader audience. We can expect less focus on issues that are of concern to a narrow slice of voters and more on the economy, health care, the national debt, education, the war in Afghanistan and other concerns of the entire electorate.

President Barack Obama, meanwhile, will no longer be able to sit back and watch the Republicans drag each other down. Instead, he will be forced to clearly articulate his priorities and vision for a second term and to defend his record from his first.

The Republican primary season has certainly been entertaining. It has likely also produced a stronger candidate, one more tested and thoroughly vetted than would have been the case if the nomination had been settled quickly. But it's time for the large majority of Americans who don't vote in Republican primaries or attend Republican caucuses to become directly engaged. America has a big decision to make in November about the direction of the country. The sooner we begin focusing on it, the better.