No one doubted that state Sen. Barack Obama (search) had the credentials to run for the U.S. Senate, but many questioned his chances against a field that includes a multimillionaire political newcomer and a son of a Democratic power broker.

Now the Ivy League-educated law school instructor has taken the lead in a seven-way Democratic primary, with the help of a divorce scandal that has enmeshed a wealthier rival and an enormously effective TV ad.

"Our theory has always been that if voters knew who I was, my track record of seven years in Springfield, and the work that I've done on issues like children's health and tax justice, the people would be responsive," said Obama, a quiet campaigner who was the first black president of the Harvard Law Review.

The race to fill the seat being vacated by Sen. Peter Fitzgerald (search) is shaping up to be one of the year's most high-profile and expensive campaigns in the battle over who will control the narrowly divided Senate. Democrats consider it a crucial seat if they are to regain the majority.

Seven of the 15 Republicans and Democrats on the ballot are millionaires, led by former futures trader Blair Hull (search), who has spent about $29 million of his fortune so far.

Hull was once the Democratic frontrunner, but his campaign began to crumble after he became embroiled in a divorce scandal that included allegations he verbally and physically abused his second wife during his second marriage to her.

Hull has taken out full-page newspaper ads and run a television commercial to defend himself against the allegations. He has accused special interests and "insiders" of orchestrating attacks on him, and had his first wife vouch for his nonviolent nature.

Reports that Hull had rarely voted in previous elections, including the 2000 presidential election, also hurt his attempt to show himself as statesmanlike.

At about the same time, Obama began running ads that portray him as the successor to the late Sen. Paul Simon, an icon of Illinois politics.

The spot features Sheila Simon talking about how her father and Obama had worked together on important issues. Video of the senator in the early part of the commercial gives way to video of Obama in similar situations. The ad closes with a black and white photo of Simon and Obama together.

It could have turned out morbid and distasteful just months after Simon's death, but instead has resonated deeply with voters, campaign manager Jim Cauley said Wednesday.

Analysts say the ad, Hull's demise, and Obama's success in rallying the black vote has helped Obama emerge as the new frontrunner.

"Part of it is predictable and part of it is fortuitous," independent political consultant Don Rose said in explaining Obama's rise. "The black vote is starting to consolidate. That is predictable. The fortuitous part is the decline of Blair Hull."

Obama's momentum has helped him counter the campaign of state Comptroller Dan Hynes, whose ties to organized labor and his father, a powerful Chicago Democratic committeeman and former state Senate president, made him an early favorite.

Rose said Hynes' ties to the Chicago Democratic organization could be worth enough votes to win a close, low-turnout election, but that Obama looks close to unbeatable.

Hynes isn't so sure.

"Elections are crazy things. Anything can happen in a week," he said.

Any Democrat is almost certain to go into the fall campaign against a novice — only one of the seven Republican candidates has any government experience, and he is trailing badly in polls.

Jack Ryan, a photogenic millionaire who gave up investment banking to teach at an all-black high school and was once married to actress Jeri Ryan, has held a commanding lead among GOP voters. Some of his rivals have attempted to force Ryan to open records from a custody case, but he has insisted they are sealed for the sake of the couple's young son.

Obama has a compelling story: His father was a member of Kenya's Luo tribe, born on the shores of Lake Victoria. He met Obama's mother, who was white, when both were students at the University of Hawaii.

When Obama was 2, his father left the family, returning to Kenya, where he eventually became a senior economist in the Ministry of Finance.

Obama grew up to receive degrees from Columbia and Harvard universities, and now is an instructor at the University of Chicago Law School.

If he is the nominee, Obama will probably be the favorite. Illinois has voted solidly Democratic in the past three presidential elections and is considered likely to do so again.

If elected, he would become only the third black candidate popularly elected to the Senate in the past century. One of them was former Illinois Sen. Carol Moseley-Braun, who was defeated by Fitzgerald after one term.