If you want to understand the difference between Democrats and Republicans, consider carefully the sad tales of two moderates.

Both moderates are U.S. senators, from different parties. Both have undergone brutal primary challenges. Both are deeply unpopular with their party’s base. Both risk defeat this coming November. With that said, the similarities end.

On one hand, Lincoln Chafee, a Republican incumbent from Rhode Island, committed any number of heresies against the conservative establishment. He is a liberal northeastern Republican who is pro-choice and gay-friendly. He abhors Bush tax cuts. He gets higher marks from the American Civil Liberties Union than some Democrats. He publicly declared in 2004 he wasn’t going to vote for President George W. Bush ’68. (Instead, he voted for George H.W. Bush ’48. One suspects vote counters might have gotten confused.) He is the Republican every other Republican loves to hate.

Yet when Chafee faced a challenge in the Rhode Island primary last week from a dyed-in-the-wool conservative and it began to look as though Chafee might lose, the national GOP pulled out all the stops. Rhode Island Republicans were bombarded by mailings and TV ads as the Rove-ian conservative machine launched a state-of-the-art GOTV operation in defense of its single-most unreliable cog.

Of course, one should not believe the GOP actually values Chafee’s opinion. It doesn’t. The man’s a laughingstock within the Bush White House, chortled at as a hapless throwback to a long-departed era of Rockefeller Republicanism. Yet when push came to shove, Republicans knew they needed Chafee’s body to prop up their faltering congressional majority. Rove and company swallowed their bile and managed not to ignore Chafee just long enough to win the primary for him.

Now, consider the other moderate: Joe Lieberman. Right off the bat, my analogy isn’t fair. Whereas Republicans are in eminent danger of losing the Rhode Island seat, Democrats are in absolutely no danger whatsoever of losing in Connecticut. Chafee was important to the GOP’s goal of retaining the Republican majority. Lieberman is not necessary to the Democratic goal of dismantling it.

Lieberman is a rare species among Democrats: a statesman seen as serious, bipartisan and deeply religious, with strong national security credentials; well-liked and oft-praised by Republicans, not to mention independents.

I don’t really want to add to the cacophony swirling around the Lieberman-Lamont primary. There is a lot to say about it, and much has been said. Yes, of course, there are many valid reasons for liberals to dislike Lieberman. Yes, his continued support for the invasion of Iraq is galling to many on the left. Yes, you can even say (though I would disagree) that there is something irritatingly sanctimonious about his public persona.

But you also have to allow that Lieberman has performed vital services for Democrats in the past. Why? Because he loudly espoused some of Clinton’s most successful policies before even Clinton decided to support them.

Take welfare reform. The Welfare Reform Act of 1996 was one of the great successes of the Clinton administration, in both political and policy terms. It took hundreds of thousands of welfare recipients off the welfare rolls and put them back to work, and it was instrumental to Clinton’s subsequent reelection. Lieberman was championing welfare reform in the U.S. Senate back in 1991, long before Clinton signed that law.

Or take Kosovo. Lieberman supported launching air strikes against Slobodan Milosevic’s regime well in advance of Clinton’s decision to actually do it. Clinton’s strikes were instrumental in removing an evil regime from power without the unrelenting bloodbath that seems to accompany regime change these days.

Was Lieberman’s stance on Iraq misguided? Yes. Did it justify the onslaught we just observed in this state from the rank and file members of the Democratic Party? The deeply personal assaults by Markos Moulitsas and all the shrill fanatics from the blogosphere? After 18 years of service to the Democratic Party, after helping to lead the Democrats to some of the most successful policies in a generation, did Lieberman really deserve to be unceremoniously chucked out of his own party?

If you view Lamont’s primary victory as roughly akin to the second coming, ask yourself: What is the moral of this tale of two moderates?

Just this irony: Republican moderates are carefully propped up come election time, then completely shut out of the policy-making process. Democratic moderates are dealt with exactly in reverse: They have great moderating influence on policy but are hated and reviled by our left wing. We as a party are not able to cynically swallow objections to their politics long enough to keep them strategically in place when we need them politically. One approach is good for formulating policy. The other is good for winning elections.

Roger Low is a senior in Branford College. His column appears on alternate Thursdays.