The once unambiguous underdog, Barack Obama is now the decisive frontrunner for the Democratic nomination. While Obama needs to focus all the time he has on winning delegates, it seems unfair to allow John McCain to be the sole recipient of running-mate predictions. So who would Obama pick as his vice presidential candidate? Let the conjectures begin!

Delaware Sen. Joe Biden: Not immune from making the occasional gaffe, and often accused of being long-winded and pompous, the selection of Biden for the ticket could be a risky bet. It could also have some heavy payoffs, especially with regard to Biden’s reputation as a foreign policy heavyweight. But perhaps most importantly, Biden has a no-bullshit tenacity, which could aid him perfectly in playing attack dog against McCain while Obama stays out of the mud.

New Mexico Gov. Bill Richardson: While just weeks ago Richardson was running for president, he could never adequately dodge speculation that he was always aiming for the number-two slot. As veep, Richardson’s years as a congressman, high-level Clinton cabinet official and now governor could help blunt Republican assertions of Obama’s “lack of experience.” He could also help consolidate the Hispanic vote – a constituency necessary for any Democratic nominee to carry. Unfortunately for Richardson, he stumbled his way through his presidential campaign with a series of embarrassing blunders, and that could seriously blunt his chances of being asked to join the ticket.

Virginia Sen. Jim Webb: With an unrivaled pugnacity, Webb could – like Biden – be exceptionally effective when going on the attack. Webb’s also a freshman senator, which would complement Obama’s “politics of change” theme. Having served as a soldier in the Vietnam War and as Ronald Reagan’s Secretary of the Navy, he would demonstrate an appeal to Republicans. Webb possesses the national security chops and combat cred needed to make him an effective surrogate for pointing out the reckless stupidity of McCain’s lust for war. And, of course, Webb could put red state Virginia in play.

Virginia Gov. Tim Kaine: Having endorsed Obama before it was fashionable, Kaine’s loyalty should boost his chances of being selected for the running-mate gig. Unarguably competent, Kaine’s career has been marked by his successful appeal to Republican voters and his ability to work with Republican lawmakers – an attractive characteristic, and one nicely complementing Obama’s “post-partisan” narrative. Like Webb – although a more popular statewide figure – he could help push Virginia into the Democrats’ column in November.

Missouri Sen. Claire McCaskill: This freshman senator has already earned a reputation for a sharp intellect and a firm combativeness frequently displayed during committee hearings. McCaskill has worked on legislation with Obama and endorsed him in January. Missouri is a swing state, and although she could probably push Obama over the edge in that state, a ticket occupied by two Midwesterners could potentially alienate voters in the rest of the country. Still, don’t rule out her chances just yet.

Kansas Gov. Kathleen Sebelius: Okay, not the most obvious choice for a running mate, but the Democratic governor of Kansas is tremendously popular. She’s been able to earn incredible Republican affection in one of the most conservative states while still championing traditionally progressive issues like abortion rights. Sebelius delivered the Democrats’ response to George Bush’s State of the Union speech this year in a widely panned rebuttal. This, coupled with the fact Kansas only carries six electoral votes, probably hurts her chances at being asked to join the Democratic ticket. However, a Sebelius pick would be a wild card, and one would be unwise to casually dismiss her.

The above list is by no means complete, and the choice by Obama and his aides will undoubtedly be a difficult decision. There is also, of course, the ever-present specter of Hillary Clinton hovering over the process. While Obama’s camp may dislike Sen. Clinton, the potential for media-fueled momentum could push Clinton onto the ticket. My prediction is the longer the vice presidential speculation lingers after the primary, the more likely the chances Clinton will be pressured onto the ticket. While it would be presumptuous and arrogant for Obama to announce a running mate before the primary season is over, once it does end, expect a sans-Clinton ticket to be announced sooner, rather than later.