In the coming phase of caucuses and primaries, Iowa, New Hampshire and South Carolina matter disproportionately — even though they’re flawed mirrors of America — because they grant or deny momentum and its attendant donations and media coverage.

And on the far side of the parties’ nominating conventions, another cluster of states — the ones said to “swing” — crowd out all the others. This cluster has shrunk over the last four decades, as most other states turned inalterably blue or red in presidential elections.

It used to be that dozens of states were up for grabs. In 1976, 20 of them, including California, New York and Texas, were decided by 5 percentage points or less.

But in 2012, only five were: Florida, Ohio, Colorado, Virginia and North Carolina. Only a few more were fiercely contested.

A postelection analysis by The Washington Post showed that almost three-quarters of the $896 million of television advertising for the Obama-Romney race was spent in five states (Florida, Ohio, Colorado, Virginia and North Carolina) with just 17 percent of the population. It was as if the other 250 million Americans didn’t count.

More than half of the $896 million was spent in just Florida, Ohio and Virginia.

These advertising patterns were examined further in a 2013 study by Robert Richie and Andrea Levien of the advocacy group FairVote, which exposes flaws in how our elections work. Richie and Levien looked at two other swing states in 2012.

“While the Obama and Romney campaigns and their allied groups spent an average of more than $30 per eligible voter in New Hampshire and Nevada after April 10,” Richie and Levien wrote in the Presidential Studies Quarterly, “they spent less than one cent per eligible voter in 34 states that together represented two-thirds of all eligible voters.”