By Kevin Connolly
BBC News, Raleigh, North Carolina
Mrs Clinton has targeted her message at blue-collar voters in Indiana
There was a time in the early spring when Democratic primaries were looming on America's horizon and then receding from its rear-view mirror without exciting much comment - remember Maine or Vermont anybody?
But with a less than a month to go before the last of the states votes in early June, every battle counts.
The candidates are running out of road, they are still running neck-and-neck and very soon now the race will be settled one way or the other.
The race is so close that it is not just the battles that count - the minor tussles do as well.
Last weekend's primary on the US Pacific territory of Guam was little more than a skirmish (it sends eight delegates to the Democratic Convention in the Summer and they only have half a vote each), but in the context of the 2008 election it nevertheless merited quite a bit of coverage.
It helped to sum up how the race between Barack Obama and Hillary Clinton is going - the island 4,000 miles adrift of Honolulu has a population of about 175,000 and Mr Obama won by a margin of just seven votes.
The primaries in Indiana and North Carolina are the last two really significant electoral prizes on offer.
They offer 187 delegates between them out of the 2,025 a candidate needs to win the nomination, so 6 May is the last date on which there could be a significant shift in the electoral arithmetic.
For quite a while, Mr Obama has held a lead in terms of the overall number of valid votes cast, states won and delegates secured, but Hillary Clinton's real achievement over the last few months has been to make sure that this race is not simply about arithmetic, as it really should be.
The big danger for [Mr Obama] is that by failing to match her feisty tone, he might strike some Democrats as weak
Her refusal to be beaten, or to acknowledge that her opponent is the likelier winner is nothing short of extraordinary.
It is hard to remember now that in the run-up to primaries in places like Texas and Ohio there was talk that she should simply acknowledge that she could not win, and pull out of the race to allow Mr Obama to prepare to fight John McCain in November.
How hilarious that suggestion seems in retrospect.
To Senator Obama, his rival must be starting to seem like one of those nightmare characters in a horror movie who takes bullet after bullet and just keeps on coming, inexorable, indestructible and terrifying.
Even if she does not win the most delegates via the primaries, she will be arguing to super-delegates (senior Democrats who control 20% of the votes for the nomination) that they should make her the candidate because of her sheer unsinkability.
Senator Clinton has thrived in her new role as a kind of champion of the little guy, the working single mother, the breadwinner who loses a job, and the family hurting because the prices of food and fuel are going through the roof.
Governor Mike Easley of North Carolina is no Jack Kennedy or Winston Churchill but he captured a certain quality in Hillary Clinton when he said she made Rocky Balboa (hero of the corny boxing movies) "look like a pansy".
One of her trade union backers described her as having "testicular fortitude" (think about it), and another compared her favourably to her "Gucci-wearing, latte-sipping" opponents.
Her best trick has been to portray herself as a kind of "blue-collar gal" fighting an elitist, out-of-touch Ivy League snob - pretty good going when you consider that she is a wealthy, well-connected former First Lady and Mr Obama was raised with the help of food stamps.
Will the voters deliver a blow to Mr Obama's electoral prospects?
He has tried to continue running the same old cool, measured, inclusive campaign based on the idea of changing the way business is done in Washington, but the big danger for him is that by failing to match her feisty tone, he might strike some Democrats as weak - maybe too weak to fight the Republicans later in the year.
The issues around which those contrasting styles have crystallised this time around are Iran's nuclear programme, and the question of whether or not to offer American motorists a "holiday" from Federal Gasoline Tax during the summer holiday season.
Senator Clinton says a break would help working families - Mr Obama says it is an irresponsible gimmick.
Mrs Clinton reminds Iran that it could face "total obliteration" at the hands of the USA if it were to launch a nuclear attack on Israel.
Senator Obama says that echoes the language of President Bush.
We will soon find out who is more in tune with the mood of Democrats in North Carolina and Indiana.
Casting a long shadow over these races has been the figure of Jeremiah Wright, the former pastor of Mr Obama's church in Chicago.
When clips of Reverend Wright's old sermons popped up on the internet (remember "God Damn America"?), critics began casting doubt on Mr Obama's judgement and even his patriotism.
There is no doubt the Wright row has damaged Mr Obama - and he is about to discover by how much.
The likeliest outcome of this latest round is victory for Hillary Clinton in Indiana, and a win for Barack Obama in North Carolina - but in both states in recent weeks, any movement has tended to go towards her rather than him.
She has said Indiana and North Carolina taken together could be a "game changer", but the truth is they probably will not be.
Once again the spoils will be divided, and the race will continue... but Hillary Clinton must be feeling that while time may be running out, her momentum is not.