Despite Hillary Clinton's impressive win in Pennsylvania Tuesday, there is virtually no scenario in which Clinton can win the Democratic nomination. Barack Obama is the almost-certain choice to become the Party's nominee -- but he will face far more hurdles on his path to the presidency than he has overcome in Democratic primaries.

Obama's record of accomplishment is slim. He served two terms in the Illinois legislature, where he did almost nothing to distinguish himself. He won his race for the U.S. Senate after the first Republican nominee had to resign over a personal scandal and the second nominee was a two-time losing senatorial candidate from another state. He has spent nearly half his time as a U.S. Senator running for president and has scant legislative achievements to his credit.

What Obama does have is a sharp mind, a gift for inspiring rhetoric, and a talent for raising lots of money. But he also has a propensity to choose his friends and allies poorly and to be unwilling to extricate himself when those relationships turn troubling. The Rev. Jeremiah Wright's racist and anti-American diatribes will continue to haunt Obama, who said he could no more disown Wright than he could his white grandmother, whom he blamed for occasionally uttering racially insensitive remarks.

But Obama's relationship to William Ayers and Bernadine Dohrn, members of the 1960s domestic terrorist group the Weather Underground, may prove more vexing yet. The Weather Underground bombed the Capitol, the Pentagon, and New York City police headquarters in the 1970s, and Ayers claimed to have personally participated in the bombings.

Ayers and Dohrn threw a fundraiser for Obama in 1995 as he began his political career, and Obama and Ayers served on a left-wing charity board together for a number of years and remain friends. It is clear from Ayers' memoir, "Fugitive Days," why Ayers would be attracted to almost any young black politician on the left; it is far from clear, however, why Obama would find Ayers an appealing ally.

In a review of Ayers' book in the New York Times, Brent Staples skewers Ayers for his patronizing attitude towards blacks. Staples, who is black, writes that Ayers described his early days as a time when he and his fellow Weathermen "sing Negro spirituals and eat chitterlings with the natives, whose intelligence and industry they find surprising." According to Staples, Ayers "is mesmerized by the incendiary violence of the 1966 race riots. 'By that time,'" Staples quotes Ayers explaining, "'I ... thought I was black.'"

Staples goes on to describe that "(w)hen the Weathermen move underground, (Ayers) likens the group to 'black Americans who must know everything about the dominant culture while remaining ... invisible to that culture.' When the group blows up a building, the act is cast as revenge for the power structure's ruthless attacks on the 'black struggle.'"

Obama has said of his relationship to Ayers, "the notion that ... me knowing somebody who engaged in detestable acts 40 years ago, when I was 8 years old, somehow reflects on me and my values, doesn't make much sense." But Ayers published his memoir in 2001 after he and Obama had become friends. And it was in a Sept. 11, 2001, New York Times article about the book that Ayers said, "I don't regret setting bombs," adding, "I feel we didn't do enough." And when asked by the Times whether he would do it all again, Ayers said, "I don't want to discount the possibility."

Most Americans know very little about the young senator from Illinois. He speaks eloquently about his love of America, so why does he gravitate towards those whose hatred of America is even more palpable?

It's a fair question, which voters will be asking in November -- and it has the potential to trump whatever electoral advantages Democrats now think they have. The Democrats are counting on Americans' disillusionment with the Iraq War and worries about the economy to propel them to victory no matter whom they nominate. But it may not be as easy as it looks.