With respect to the future party composition of the U.S. Senate, the Democrats' top goal is to emerge unscathed from the 2010 elections and preserve their numerical dominance, which (at least on some issues) now stands at 60 seats. But, in the tradition of Democrats behaving like Democrats, they appear determined these days to make life difficult for themselves in a number of states, by waging the kind of intramural strife that drains party money, divides the partisans, and provides ammo to the Republicans.

Pennsylvania, of course, is Exhibit A, as evidenced yet again by the latest skirmishes featuring Arlen Specter and Joe Sestak, combatants for the '10 Senate Democratic nomination. The more they try to outflank each other on the left, as they vie for Democratic votes in the Democrats-only primary scheduled for May, the more they potentially cede the center to Pat Toomey, the conservative Republican who will meet the Democratic victor next autumn.

In the early months of 2010, Specter-Sestak will be the nation's marquee Democratic smackdown. But there will be other Senate intramurals. Two Democrats are already clawing at each other in Kentucky, each vying for the party nomination and the opportunity to snatch the Senate seat being vacated by conservative Republican Jim Bunning. Two Democrats are fighting each other in Illinois, where another Senate seat is open (thanks to the imminent departure of Democrat Roland Burris). Two Democrats are poised to battle each other in Colorado, where the primary isn't staged until August, thus giving the party very little time to reunite for the autumn finals in a swing state.

Granted, the Democrats are probably less fractious these days than the Republicans, who appear to be enmeshed in some sort of ideological identity crisis (as evidenced by the GOP's current Florida Senate intramurals, outlined in this space one week ago). Yesterday, in fact, the Republican National Committee agreed to consider a resolution imposing a purity test on all party candidates, potentially requiring them to swear fealty to a minimum of eight Reaganesque principles (including "lower deficits and lower taxes" and "opposing amnesty for illegal immigrants," - a typical example of the GOP's willful amnesia, given the fact that Ronald Reagan raised taxes a number of times, racked up deficits, and gave amnesty to illegal immigrants). In the months ahead, this proposed purity test is likely to further ratchet up internal Republican tensions.

But such tensions are to be expected, because the GOP is the out party. What's the excuse for the in party? The two Kentucky Democrats are currently accusing each other of being liars, sleazebags, flunkies of the natural gas industry (the enemy of Kentucky coal), and tax cheaters - thus supplying the '10 Republican opponent with all kinds of mud to hurl at whoever wins the primary.

The Illinois situation is equally dire. This is the Senate seat once filled by Barack Obama; then it went to Burris, with all his baggage. When appointee Burris agreed not to run on his own, Obama and his lieutenants tried to tap state attorney general Lisa Madigan, but she said no. The vacuum has now been filled by two underwhelming candidates, Chicago official David Hoffman and state treasurer (and apparent frontrunner) Alexi Giannoulias. Hoffman has been busy trying to paint Giannoulias as a sleazy banker who can't win in November '10 - because, among other things, the Giannoulias family bank (where he worked prior to becoming state treasurer) once made loans to convicted felon/fundraiser Tony Rezko, and that he oversaw various bank loans to convicted felons with alleged mob ties. If Giannoulias wins the primary in February, that kind of material would be grist for the Republicans as they seek to capture a Senate seat in this normally blue state.

And speaking of blue states, we have the latest Democratic primary skirmishings in Pennsylvania. Late last week, chameleon Senator Arlen Specter, mindful of his need to shed his old Republican plumage and move sharply to the left, publicly morphed into a dove and announced that he was opposed to sending any new troops to Afghanistan. He then said, "When you have Congressman Sestak calling for an increase, a major increase, I think his view would be in the minority" - meaning, of course, the minority of grassroots Democratic sentiment. (Sestak has said that he supports "a measured increase" of troops.)

But Sestak, determined not to be outflanked on the left, struck back yesterday by tying Specter to Sarah Palin - a useful exercise, since the typical Democratic primary voter sees her as a joke. The new Sestak video shows Specter on the stump with Palin in 2008, and telling Chris Matthews on Hardball that the McCain-Palin ticket was "the better choice." A Sestak-sponsored website quips, "That kind of loyalty deserves a place in (Palin's) bestseller."

If Specter and Sestak keep veering leftward during their primary contest, it could create an opening for Toomey, the Republican candidate, to seize the center. Toomey is already working to downplay his fiscal and social conservativism in favor of a more "center-right" profile, and he certainly has the space to pull it off, given the fact that (according to polls) 53 percent of Pennsylvanians have never even heard of him. So put Pennsylvania on the list with Illinois as a potential blue state pickup for the Senate GOP.

But, with respect to this intraparty strife, there's at least one short-term upside for the Democrats: It's helping to keep Blanche Lincoln in line.

The centrist Democratic senator from Arkansas voted to send health care reform to the floor on Saturday night; her eleventh hour vote was privotal. Had she voted with the Republicans, the odds of her facing a 2010 primary challenge from the left would have exponentially increased. Indeed, Arkansas Lt. Gov. Bill Halter seems poised to pounce if she sides with the Republicans to block health reform in the crucial votes to come. (Halter has already implicitly warned Lincoln, warning on TV last week that Arkansans "really want action" on health reform. "They're growing very impatient with all the talk.") And grassroots liberal groups, notably moveon.org, are ready to finance a primary challenge if Lincoln stiffs the party leaders. In these polarized times, perhaps it can best be said that both parties have their purity tests.