As the closely fought Democratic presidential contest between Hillary Clinton and Barack Obama moves deeper and deeper into the primary season, there is a growing sentiment that the nomination should go to the candidate that ultimately wins the popular vote.

Fair enough. Ever since the current primary-dominated era of nominations began in the 1970s, every Democratic and Republican nomination has gone to the candidate who received the most votes in the presidential primaries. The last nominee who was not his party's top primary vote-getter was Democrat George McGovern in 1972. And that was the last election in which less than half the states held primaries.

In recent decades, the aggregate nationwide vote has been won in decisive fashion by the ultimate nominee. But so far this year, that has not been the case with Clinton and Obama. There are different ways to count the Democratic popular vote. And while Obama comes out of the Feb. 19 voting leading in all of them, Clinton could still vault on top with a succession of strong showings in the big primary states that vote directly ahead.

Limit the tally to sanctioned Democratic primaries, and Obama presently enjoys a lead of more than 700,000 votes. Count the ballots from all primaries, including the non-binding votes in Florida and Michigan that Clinton dominated, and his advantage drops to barely 100,000 votes.

But add to this mix the reported vote for the candidates from caucus states, where Obama has had the upper hand, and his lead grows to more than 300,000 votes. And if one wishes to delete the unsanctioned Florida and Michigan results, and limit the tally to sanctioned primaries and reported caucus votes, Obama's lead swells to more than 900,000 votes.

To compound the complexity, the actual presidential preferences of attendees was not tallied in some of the most highly-publicized caucus states, including Iowa, Nevada and Washington. There, the state parties presented the vote in terms of local delegates elected and did not conduct a primary-like tally of participants. If such a tally is attempted by extrapolation--multiplying the delegate percentages for Clinton and Obama times the voter turnout--the Illinois senator would add roughly another 100,000 votes to his total.

What then is the "popular vote"? It would probably take a judge wiser than Solomon to declare a hard and fast winner at the end of the primary season in June if different formulations of the popular vote produce different winners.

Democrats can hope it does not come to that, and they have history on their side. The pattern over the last two decades has been for candidates to trade victories over the first few weeks of the primary season before one candidate catches hold and scores a long string of wins that puts the nomination away. By the end of the process, there has been a clear winner and the popular vote is regarded as little more than an interesting curiosity.

Figure 1. Presidential Nominations in the Primary Era Have Always Gone to Top Vote-Getters

It was in the presidential election of 1976 that more than half the states held presidential primaries for the first time. And starting with that election, both the Democrats and Republicans have nominated the candidate who amassed the highest number of primary votes. Not since Democrat George McGovern back in 1972 has a presidential standard-bearer not been his (or her) party's top vote-getter in the primaries. As for the general election, it has usually, but not always, been won by the candidate who had been most dominant on his side of the primary ballot. A conspicuous exception was 1992, when Bill Clinton won the White House after a primary season that was much more of a struggle for him than the Republican incumbent, George H.W. Bush. Nominees in contested nominating races since 1972 are indicated below in bold; incumbents are noted with an asterisk (*). Primary results include contests in the 50 states and the District of Columbia, but not territories.

Note: A pound sign (#) indicates that Al Gore lost the electoral vote in the 2000 presidential election but won the popular vote.

Source: Race for the Presidency: Winning the 2008 Nomination (CQ Press).

That process of consolidation could be happening now, as Obama has swept every delegate-selection event since Feb. 5--a total of five primaries, four caucuses and overseas votes involving the Virgin Islands and Democrats Abroad. Just since Super Tuesday, he has outpolled Clinton by more than 800,000 primary votes.

But the Clinton-Obama campaign has been anything but predictable. Just when it appears one of the candidates is about to pull away, the other mounts a comeback, with eyes focused right now on Clinton.

It has been in the big states where she has enjoyed her greatest success thus far. And on the primary calendar in March and April, big states loom large. Ohio and Texas hold primaries March 4, with Pennsylvania voting on April 22. In diverse states this size, Clinton has already showed her mettle, carrying California by more than 400,000 votes, New York by more than 300,000, Massachusetts by nearly 200,000 and New Jersey by more than 100,000 votes.

To be sure, Obama has had some large-scale successes of his own. In sweeping the primary in his home state of Illinois, he rolled up a margin of more than 600,000 votes. Obama also carried Georgia by 375,000, Virginia by almost 280,000 and Maryland by more than 200,000 votes. But the caucus states, which have fueled much of his recent success, have almost all completed their first-round voting, the level where a popular vote is taken and delegates are initially apportioned. Only Texas (where caucus action will choose about one-third of the elected delegates) and Wyoming remain to hold caucuses, as well as a handful of territories led by Puerto Rico.

The upshot is that the Democratic popular vote, no matter how one wishes to count it, could remain close, complicated and controversial for some time to come. And who's ahead at any particular moment, Barack or Hillary, could be in the eye of the beholder.

Figure 2. Democratic Popular Vote Leader: It Depends on the Count

Decisions, decisions, decisions. When it comes to a popular vote count in this year's Democratic presidential race, there are a variety of formulations that can lead to a number of different outcomes. Heading toward the big March 4 primaries in Ohio and Texas, Barack Obama leads in all the basic formulations for tallying the popular vote. But his advantage in some are much narrower than others. And it is arguable that Obama's lead in all of these formulations can still be overturned by a series of strong Hillary Clinton showings in the big states left to vote. The tallies are from all states that voted through Feb. 19 plus the District of Columbia.

Note: Vote totals are based on a combination of official and nearly complete but unofficial returns from Democratic primaries and caucuses, as of Feb. 21. Votes are taken from the web sites of state election boards for primaries and Democratic state parties for caucuses. An asterisk (*) indicates that Obama leads the non-binding Democratic primary in Washington but the race remains undecided. Because Washington Democrats held both a caucus and non-binding primary, there is potential for a "double count" in that state.

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Views expressed in this column are those of the author, not those of Rasmussen Reports.