Delegate Estimates Updated at 5:23 p.m. ET

Mitt Romney won six of 10 states up for grabs on Super Tuesday and built up a commanding lead among delegates who will officially select the party's nominee at this summer's Tampa convention.

In total, Romney won 217 delegates Super Tuesday, almost half the total 437 at stake Tuesday, and more than his competitors combined. If Romney had fallen short of 200, it wouldn't have looked good. If he'd climbed toward 300, his aura of inevitability would have glowed even brighter.

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But the former Massachusetts governor is not yet a sure thing for the nomination and he must win 47 percent of the remaining delegates before he can rightly be called the presumptive nominee. The math of the GOP nomination lays groundwork for a delegate fight that could extend well into the summer and even potentially leave Romney without the magic number 1,144 delegates needed to clinch the nomination.

Romney scored victories in Virginia, Vermont, Ohio, Idaho, Alaska and Massachusetts. Rick Santorum emerged the victor in Oklahoma, North Dakota and Tennessee. And Newt Gingrich won his home state of Georgia. Ron Paul came up empty-handed, again.

Romney now leads all candidates with 401 delegates, according to the latest ABC News estimate. Rick Santorum follows with 177, and Newt Gingrich (106) and Ron Paul (45) trail behind.

There are a total of 2,286 delegates up for grabs during the primary and 729 have already been estimated, about 31 percent.

That leaves 1,557 outstanding delegates and Romney will have to win 743 - 47 percent - to reach 1,144.

The calendar could present Romney with some problems. The next contests focus on the South, where he is thought to be weak against Santorum. And the main delegate prizes are more than a month away. New York holds its primary in late April. Texas is likely to hold its primary in late May. And California Republicans do not vote until June.

With most of the votes already cast, the logistical nightmare of awarding the rest of those 437 delegates could take far longer to unravel.