Is there room in the Republican Party for genuine moderates? Truth to tell, the GOP can’t decide. More precisely, it’s deeply divided over whether it should allow any divisions in the party at all.

That’s why the brawl in a single congressional district in far upstate New York is drawing the eyes of the nation. Conservatives are determined to use the race to prove that there is no place in the party for heretics, dissidents or independents.

President Barack Obama set up the fight by nominating the district’s former representative, John McHugh, as his Army secretary. Maybe Obama is as fiendishly clever as his more paranoid opponents believe him to be.

When local Republicans picked a moderate, Assemblywoman Dede Scozzafava, as their candidate for the Nov. 3 contest, many on the right rebelled. They are backing a third-party conservative, Doug Hoffman, and he may well drive Scozzafava into third place. For the moment, at least, polls show that Bill Owens, the Democratic candidate, has jumped into first place on the split.

Demonstrating just how far right some Republicans have moved is the fact that former House Speaker Newt Gingrich is on the moderate side of this civil war against his old nemesis Dick Armey, who served under Gingrich as majority leader.

Gingrich, who backs Scozzafava, always understood that he would never have become speaker without help from Republican moderates. Armey prefers ideological purity and, like fellow members of the tea party movement, is supporting Hoffman.

The GOP’s battle of Plattsburgh and Oswego underscores a little-noticed truism of American politics in 2009: While the Democrats are a coalition party uniting moderates and liberals, Republicans threaten to become a party of the right and only of the right. That means (as we are seeing with health care) that many of the big arguments take place inside the Democratic Party.

Democrats won their majority in Congress by uniting and firing up their base (George W. Bush helped a lot) and by winning over moderates and independents, often by running moderate candidates in conservative districts. These candidates were typically to the left of the Republicans on economic issues, but to the right of, say, Berkeley and Cambridge.

In the meantime, middle-of-the-road voters who had populated the moderate Republican heartland, notably in suburban areas of the Northeast and Midwest, shifted steadily Democratic, turned off by the increasing dominance of Southern conservatives in the party of Lincoln.

Such voters threw solid Republican moderates out of office — among them Connie Morella in Maryland, Jim Leach in Iowa and Chris Shays in Connecticut. In the Maryland, Iowa and Connecticut cases, this did not occur because those three champions of the middle way were disliked but rather because they came to be seen as enablers of a right-wing congressional majority.

If you take conservatives aside quietly, they will often acknowledge all this. For next year’s Senate races, party leaders have already welcomed the emergence of moderate candidates, notably in contests for the seats once held by the current president and vice president of the United States.

Republicans have a decent shot in Delaware because Rep. Mike Castle, who leans toward the center, has decided to run for Joe Biden’s old seat. In Illinois, Rep. Mark Kirk, despite some opposition on the right, offers the Republicans their best chance of capturing the office Obama gave up to pursue other interests.

But this strategy won’t work if staunch conservatives insist that in the crunch, there is no space in the party for anyone to Dick Armey’s left. “We win when we are us,” Armey told the right-wing blog Red State in explaining his support for Hoffman. It’s a revealing statement. The “us” Armey has in mind isn’t big enough to encompass people like Castle and Kirk, let alone the scores of moderate suburbanites who once felt comfortable in the Republican Party of George H.W. Bush.

In fact, “us” is getting to be a small, comfy group. The Washington Post-ABC News poll this week found that only 20 percent of adults identify themselves as Republicans, the lowest single number in Post-ABC polls since 1983. Only 19 percent had confidence in congressional Republicans “to make the right decisions for the country’s future.” Even congressional Democrats got 34 percent on that question, and Obama scored 49 percent.

With the moderate Scozzafava’s support in apparent collapse, Hoffman has a chance to win next month. Right-wingers would cheer, but such a result would be bad news for the GOP. A Hoffman victory would keep Republicans on a path that Newt Gingrich knows is unsustainable, even if Dick Armey doesn’t.

E.J. Dionne’s e-mail address is ejdionne(at)washpost.com.