In a just-concluded conference call, Mark Penn, a senior strategist for Sen. Hillary Rodham Clinton's (N.Y.) campaign, argued that his candidate alone is positioned to beat Sen. John McCain (R-Ariz.) in the fall thanks to her past ability to fight against the Republican "attack machine" as well as her skill at neutralizing the issue of national security.

"The Republican attack machine redefines the Democratic candidate," said Penn, pointing out that Vice President Al Gore and Sen. John Kerry (Mass.) both felt the sting of the GOP efforts during their respective national bids. Penn added that while Clinton is well known in national circles, Obama is less so -- a lack of name recognition that leaves the Illinois senator open to being defined by the Republican nominee. "Hillary has withstood this process and this will make a tremendous difference if she is the nominee," he said.

The other prong of the Penn argument is that the likely nomination of McCain means that national security will again be at the forefront of voters' minds this fall, as it was in 2004.

McCain has made no secret of his proud advocacy of the surge strategy in Iraq and, in order to win the Republican nomination, he has relied heavily on his personal biography that includes five years spent in a prisoner of war camp in Vietnam.

Nominating Clinton would "block [Republicans] from playing the national security card," argued Penn, adding that Republicans have already begun to attack Sen. Barack Obama (Ill.) on his foreign policy credentials. (In an interview yesterday with Fox New Channel's Chris Wallace, President George W. Bush had this to say of Obama's foreign policy bona fides: "I certainly don't know what he believes in.")

Penn's call is the latest in a series of assertions by Clinton's camp that she is more electable than Obama in the general election because Obama has not been as thoroughly vetted as the former first lady and remains a new-ish figure on the national scene.

The Obama campaign, meanwhile, is making its own electabilty argument, releasing a memo late last week that shows the Illinois senator running stronger than Clinton in most recent national polling. "Barack Obama is the candidate best suited to win independents, play well in red states and beat John McCain in November," the memo reads.

As evidence, the memo cites recent polls from Time, CNN, Cook Political Report, Post/ABC, Fox News and Rasmussen -- all of which show Obama running slightly to considerably stronger than Clinton in hypothetical matchups against McCain. A new survey out today -- conducted by the Associated Press -- affirms that idea with Obama leading McCain, 48 percent to 42 percent, while Clinton leads McCain, 46 percent to 45 percent.

The central difference in the electability appeals by the two campaigns is temporal.

The Obama campaign argues that the way to best understand who is the more electable is to look at current polling and past results to see who leads the likely Republican nominee and who is better able to lure crucial independents to the Democratic cause. The present is what matters, says Obama.

For Clinton, it's the future that's the issue. Sure, they argue, Obama may be ahead right now, but Republicans have only begun to define him, a process that would strip away much of his independent support and leave him on the losing end of a race against McCain.

"In a general election the Republicans will spring into action and quickly, if he were the nominee, roll out his whole record," said Penn. "The kind of independent support that he has had so far would evaporate relatively quickly once he faced Republicans."

The electability argument is, at its center, dependent on how Democrats view this nominating fight.

Clinton and her team believe that the party is essentially risk-averse, a position born of the disappointing results of the last two presidential elections in which the party's nominees were negatively defined by a concerted Republican effort.

Obama's claim of electability is based on the idea that the way politics has been conducted over the past several decades need not to be the way it operates going forward. The driving force behind Obama's argument is that unlike the past several elections that have been focused on turning out the base of each party and trying to peel off just enough independents to win, the 2008 contest could well be a transformational choice in which independents and even many Republicans put aside partisanship and cast a vote for him.

Left unsaid, but of course implied, is that Clinton is far too polarizing to change the electoral math and that, if she were able to win, would do so in a squeaker.

Who's right? Well, Republicans have already begun their effort to define Obama for voters. Of late, almost every email out of the Republican National Committee notes that Obama was the most liberal Senator in 2007. (Don't forget just how damaging that same vote rating was to Kerry in 2004.)

On the other hand, it's hard to imagine Republicans not joyous at the prospect of dredging up all of the old attacks against the Clintons if the New York senator winds up as the nominee.

Either choice represents a risk for Democrats. Obama is less well known and less tested on the national stage but has shown a capacity to reach independents and Republicans that Clinton won't likely be able to match. Clinton is the more polarizing figure of the two, but what else bad could be said about her that voters haven't already heard?

Such is the nature of the choice Democrats face in the next few months.