Barack Obama gives a winning smile after taking Wisconsin
Senator Barack Obama is proving hard to stop.
Wisconsin was not just his ninth successive win since 24 states held contests on Super Tuesday early this month; more importantly, it showed that he is now eating into Senator Hillary Clinton's core support.
The make-up of Wisconsin should have worked in Mrs Clinton's favour.
Almost 90% of those voting were white, most of them working class and more than 50% of them women - in other words, the group that until now has proved to be her base.
But this time she barely won among white women, and lost ground among less educated and lower-income voters.
Wisconsin showed that it is Mr Obama who is broadening his appeal.
Wisconsin marked another important test. It is just the kind of state that the Democrats would need to win in a general election.
Hillary Clinton 17 states, 1,592 delegates
Arizona, Arkansas, California, Florida, Massachusetts, Michigan, Nevada, New Hampshire, New Jersey, New Mexico, New York, Ohio, Oklahoma, Pennsylvania, Rhode Island, Tennessee, Texas
Barack Obama 24 states, 1,723 delegates
Alabama, Alaska, Colorado, Connecticut, Delaware, Georgia, Hawaii, Idaho, Illinois, Iowa, Kansas, Louisiana, Maine, Maryland, Minnesota, Missouri, Nebraska, North Dakota, South Carolina, Utah, Vermont, Virginia, Washington state, Wisconsin
2,025 delegates needed for nomination. Source AP (includes all kinds of delegates) Q&A: US election delegates
Arizona, California, Connecticut, Delaware, Florida, Illinois, Maryland, Missouri, New Hampshire, New Jersey, New York, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, Texas, Vermont, Virginia, Washington state, Wisconsin
Mitt Romney 11 states, 251 delegates
Alaska, Colorado, Maine, Massachusetts, Michigan, Minnesota, Montana, Nevada, North Dakota, Wyoming, Utah
1,191 delegates needed for nomination. Source: AP (includes all kinds of delegates)
And exit polls showed that more voters now think that Barack Obama would be a more formidable opponent against a Republican candidate than Hillary Clinton, the senator for New York.
She is still ahead, though, on experience.
A loss in Wisconsin also bodes badly for Hillary in Ohio - another white, blue-collar state.
Ohio, along with Texas - two big states that vote on 4 March - are now must-wins for Mrs Clinton.
Once she confidently called those states her "firewall".
But even in Texas with its large Latino population, Mrs Clinton and Mr Obama are now neck-and-neck in the polls.
She cannot afford to be so sure that the "firewall" will protect her from defeat.
The question already being asked is, has the Clinton campaign badly miscalculated by focusing so much on the big states?
Her victories in California, New York and New Jersey are now distant memories.
Hillary Clinton's core support has been eroded
Mr Obama clearly has the momentum.
For the senator from Illinois, spending time money and effort in those smaller states since Super Tuesday has clearly paid off.
Mrs Clinton has also come in for criticism on the issue of negative campaigning.
Clinton advisors were quick to accuse their candidate's opponent of plagiarism in sections of speeches he has made.
And that accusation was meant to hurt by questioning Mr Obama's strongest suit - his soaring rhetoric.
But in Wisconsin, the exit polls showed that 54% of voters thought she had attacked him unfairly - by contrast, 34% accused Mr Obama of the same.
This would seem to reinforce the response from Mr Obama that "lifting" a few lines from a friend's speech was no "big deal".
But no doubt from now on, his words will undergo greater scrutiny.
That is certainly true as far as the Republicans are concerned.
John McCain is no longer bothering about his rivals, who are so far behind that he hardly needs to bother about them.
Another win in Wisconsin for Mr McCain has surely made Mike Huckabee's continued presence futile.
The McCain camp has turned its guns on Mr Obama
In his victory speech, the veteran Arizona senator turned his firepower on Mr Obama instead - warning Americans not to be seduced by his "empty rhetoric" and calls for change.
It is all but over in the Republican race - although Mr Huckabee, a Baptist preacher, has shown no sign of bowing out, and seems to have nothing better to do than continue to fight.
As for the Democrats, Wisconsin - while important - was not make or break.
But if Mrs Clinton cannot turn the tide in two weeks' time, her position looks increasingly vulnerable. The question now: will her "firewall" of Ohio and Texas hold out?