In a previous and distant incarnation, I was a feature writer, tasked with the job of identifying "trends" in our society - political trends, cultural trends, behavioral trends, all kinds. The process of identification was a very inexact science. If you had only one anecdote, that was not enough to constitute a trend. Two anecdotes, still not good enough. But if you had three anecdotes, plus some statistical backup and the likelihood of four or more...bingo, that was grist for a "good story."

Which brings us to the House Democrats, and their prospects in the 2010 congressional elections. When congressman Dennis Moore announced in late November that he would not run for re-election next year in his Kansas district, his fellow Democrats generally dismissed his retirement as a fairly unique event. And when congressman John Tanner confirmed on Dec. 1 that he too would retire, rather than seek re-election in his Tennessee district, party strategists insisted that a pair of retirees did not foreshadow a flood of other Democrats fleeing for their lives.

Then came the Dec. 9 retirement announcement by congressman Brian Baird; he decided not to seek a new term out in Washington state. Do three Democratic retirees constitute a trend? Well, maybe not, since three could be spun by Democratic leaders as a mere trickle...But now we have a fourth. Another Tennessee Democratic congressman, Bart Gordon, announced yesterday that he will retire rather than seek re-election in a seat he has held since 1984.

OK, now we do seem poised on the precipice of a trend - particularly since more House Democratic retirements are reportedly in the offing. (By contrast, the House Democrats had only two retirements in the last three congressional elections combined.)

We won't know for awhile whether the '10 trend will be minimized - indeed, some states don't require congressmen to make up their minds until the spring and summer; that's when the filing deadlines are scheduled - but it's clear that, in the meantime, Democrats will need to keep a close eye on that retirement roster. Because every time a Democrat retires, he opens up a seat. And open-seat races are often ripe for takeover. In short, the more Democratic retirees, the better the odds of the Republicans racking up major seat gains in 2010 (although the ultimate GOP goal, of taking back the chamber with a net swing of 41 seats, remains highly daunting - especially since the Republicans will have to defend at least three competitive seats being vacated by GOP retirees).

For historical perspective on the retiree factor, look no further than the 1994 election that catapulted the House Republicans into the majority and Newt Gingrich into the speaker's chair. The Democrats that year were plagued by a wave of retirements; they had to defend 31 open seats. The Republicans wound up winning 22 of those seats - nearly 40 percent of their pickups in November.

Right now, the '09 Democrats have to defend seven open seats in competitive or Republican-leaning districts (thanks to the four retirees, plus three other Democrats who are leaving to run for other offices). That number seems quite manageable - assuming that it doesn't go any higher. But it probably will. By all accounts, at least six more Democratic congressmen could opt to retire rather than face re-election in districts that generally support Republicans; the voters in five of those districts supported John McCain over Barack Obama in the '08 presidential election. (Afternoon update: one of those six Democrats, Chet Edwards of Texas, announced today that he will not retire, and intends to run again.)

John Tanner, the Tennessee retiree and a leading "Blue Dog" Democrat, is a case in point. McCain won his district by 13 percentage points; George W. Bush won his district twice. Tanner naturally denied in his announcement that his decision to retire was connected in any way with the general antipathy toward Obama in Tennessee, or the very real possibility that the voters in his district would treat him rudely as Obama's proxy. But it strains credulity to believe that these were not at least some of the factors in his thinking. So, too, with Gordon, his fellow Tennesseean.

Every midterm election is a referendum on the party in power - as the Republicans discovered when Ronald Reagan was in the White House; in the '82 midterms, at a time of 10 percent unemployment, Reagan's party lost 26 House seats. Today's Democrats would love to limit their own damage via the Reagan model; that would require motivating their own base, because right now, according to the polls, the Republican base (older, whiter, angrier) is more motivated to show up for the midterms.

But if more incumbent Democrats decide that retirement is preferable to defending their own party - if the retiree roster hits double digits in the months ahead - then the tally will more than satisfy the definition of a trend, and portend trouble for the Democrats next November.