With a week to go until Illinois voters cast their primary ballots, a new Tribune/WGN-Ch. 9 poll shows Democrat Barack Obama has surged to front-runner status while Republican Jack Ryan maintains a sizable lead in the nomination battles for the U.S. Senate.

The survey, the last polling by the Tribune prior to the March 16 election, also shows Blair Hull, the Democratic front-runner in a similar poll three weeks ago, has seen support plummet as his campaign was consumed by allegations that he verbally and physically abused an ex-wife during a messy divorce.

At the same time, attempts by Republican rivals to raise questions of character against Ryan over portions of his sealed divorce file involving actress and ex-wife Jeri Ryan have so far failed to tarnish the image of the investor-turned-teacher.

Even so, the results of the survey conducted by Market Shares Corp. of Mt. Prospect from March 3-6 indicate that support among Democrats and Republicans alike remains extremely fluid amid what is anticipated to be an election with low voter turnout. That places a premium on candidate efforts to crank out their base vote and get supporters to the polls.

The Tribune/WGN-Ch. 9 survey of 602 likely Democratic primary voters showed Obama, a state senator from the Hyde Park neighborhood, with the support of 33 percent of primary voters, compared with 19 percent for state Comptroller Dan Hynes and 16 percent for Hull. Cook County Treasurer Maria Pappas had 8 percent, former Chicago Board of Education President Gery Chico had 6 percent and health-care executive Joyce Washington and radio talk show host Nancy Skinner each had 1 percent.

On the Republican side, the survey of 580 likely GOP primary voters showed Ryan had the backing of 32 percent, compared with 11 percent for investment dealer and boutique dairy owner Jim Oberweis and 10 percent for businessman Andy McKenna Jr. State Sen. Steve Rauschenberger of Elgin had 8 percent, retired Gen. John Borling had 2 percent and Jonathan Wright, an assistant prosecutor in Downstate Logan County had 1 percent. Two other candidates each had the support of less than 1 percent of Republicans surveyed.

Reflecting both the extreme size of an unfamiliar Republican field and voter uncertainty about the candidates, fully 35 percent of GOP voters were still undecided a week from the election.

Both surveys, conducted through random, person-to-person telephone interviews with likely registered voters, have an error margin of 4 percentage points.

The results of the Democratic survey demonstrate a dramatic shift in voter opinion about Hull, a multimillionaire who has swept away all records for campaign spending in Illinois by pumping more than $29 million of his own money into the race.

The findings also indicate a rapid decline in the number of undecided voters. Three weeks ago, more than one-third of Democrats had not picked a candidate to support; now that number is down to 16 percent.

While Hull has seen his political fortunes cascade downward, Obama has seen his grow amid a spate of targeted mailings and TV ads promoting his legislative record to women and the elderly on such issues as health care.

Moreover, Obama, the first African-American president of the Harvard Law Review, has finally begun to show success among Chicago's black voters. Just three weeks ago, Obama had the backing of only 38 percent of African-American voters. Now, that figure is 62 percent.

The poll showed Obama leading Hynes significantly in Chicago, suburban Cook County and the collar counties, while Hynes and Hull are splitting the Downstate vote. Among white voters, Obama, Hynes and Hull are showing nearly equal support.

Hynes, the son of veteran Democratic politician Tom Hynes, has received the endorsement of the powerful state AFL-CIO and is counting heavily on political organization and labor support to make sure his voters get to the polls in what appears to be shaping up as a low-turnout election.

Only three weeks ago, a Tribune/WGN-Ch. 9 survey found Hull, who has advertised extensively on TV across the state, holding the edge with 24 percent backing from Democrats surveyed, compared with 15 percent for Obama and 11 for Hynes.

But in the intervening period, Hull has been put on the defensive as he tried to explain the circumstances surrounding his 1998 divorce from Brenda Sexton and an order of protection that she had sought against him. It was the second breakup for Hull and Sexton, who were married and divorced twice in a span of three years.

Records show Sexton accused Hull of directing vulgar epithets at her, threatening her life and hitting her on the shin.

Hull did not deny her claims, but he noted that prosecutors dismissed charges against him resulting from one confrontation and contended that Sexton exaggerated his behavior and sought the protection order to try to squeeze him for a bigger divorce settlement.

Sexton has denied she had any financial motivation for the protection order request and said she sought it because she became fearful of Hull and wanted him to move out.