(CNN) There's every reason to believe former Vice President Joe Biden will be the Democratic nominee outside of some statistically improbable event. Biden followed his impressive victories on Super Tuesday with another round of strong performances on Super Tuesday II, and it looks like he will continue his march toward the nomination going forward.

Candidates in Biden's position have always gone on to win the Democratic Party's nomination. Since 1988, the candidate with a delegate lead following the round of contests after Super Tuesday has taken the Democratic nomination six of six times in competitive primaries.

Some of those were close calls (see Michael Dukakis in 1988), but there's no reason to believe this will be one of those situations. Biden is up by close to 150 delegates . That lead might not seem large. Make no mistake: it is.

Remember, Democrats allocate their delegates proportionally. Once a candidate jumps out to a clear lead with a substantial number of delegates allotted, as Biden has, an opponent must start winning contests by large margins. It's not enough just to win.

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