When a demographic group gets bigger, it usually becomes more powerful politically, but there is one major exception in the United States: voters who don’t identify with either major party. The results of yesterday’s presidential primary in Michigan seem to confirm that idea, as independents were on the wrong side of both the Democratic and Republican contests. According to CNN exit polls, 51 percent of independents voting on the Democratic side chose “uncommitted” over Hillary Clinton (Barack Obama wasn’t on the ballot), but 60 percent of self-identified Democrats went for Clinton. On the Republican side, independents voted for John McCain over Mitt Romney, 35 percent to 29 percent, but GOP regulars went 41-24 for Romney.

The pattern is becoming clear. Also according to CNN, in the Iowa caucuses Obama beat Clinton handily among independents but had a statistically insignificant edge among Democrats, while Mike Huckabee carried registered Republicans but lost independents to Ron Paul. In New Hampshire, Obama and McCain won independents, but Clinton and Romney won party regulars.

That means the smart money is against Obama and McCain in the long run. They may win a few more primaries in which independents are allowed to vote, but Clinton and Romney (or, less likely, Huckabee) can probably pull ahead in the delegate race thanks to primaries and caucuses that are open only to registered party members. (See a list at CQ.com.) They include Colorado, New Jersey, New York, and Oklahoma on February 5 (“Super Tuesday”) and Pennsylvania on April 22 (in case the nominations aren’t settled by then).

So how would the growing bloc of independent voters react if they’re repeatedly trumped by loyalist Democrats and Republicans? I can think of several scenarios. (1) Independents could turn to a third candidate, such as Michael Bloomberg, in November. (2) They could lobby state legislatures to open all presidential primaries so that independents can vote in them in the future. (3) They could form a unified bloc and concentrate their vote in either the Democratic or Republican primaries, depending on the candidates. (4) They could give up the independent option and drift back into the two major parties, trying to change them from within.

Of course, there’s also scenario (5): Independents simply don’t vote.