Another clean sweep for Mitt Romney, with a resounding win in Puerto Rico's primary. But though he's narrowly tipped to win the crucial Illinois race - how come his rival has all the momentum?

Rick Santorum. (Getty)

One candidate has all the money. All the resouces. All the staffing, and top-flight organisation. It should be a cake-walk for Mitt Romney in Illinois tomorrow night, fresh from his overwhelming victory in Puerto Rico. But he just can't catch a break from those nagging doubts about his personality. There's still no sense of excitement, even in a state which should be a natural fit. And that's why Rick Santorum is still snapping at his heels.

At least there has been some good news for Mitt, as he hailed his "overwhelming victory" in the Puerto Rico primary as proof that he could make serious inroads into the Hispanic vote. "I intend to become our nominee, and I intend to get Latinos to vote for a Republican and take back the White House" he said. In 2008, Obama won a massive 67 per cent of the Latino vote, but Republican strategists are keen to win them back, over their frustration with the state of the economy.

Hispanic vote

Not that Team Obama is worried: a campaign memo confidently predicts that the GOP has already alienated Latinos: "Their extreme rhetoric on immigration... has rejected our history as a nation of immigrants and alienated millions of Hispanic voters nationally." Certainly Rick Santorum was badly affected in Puerto Rico by a comment he apparently made about their bid for statehood. He was quoted as saying residents would have to adopt English as their primary language before that could happen: he says he was misrepresented, but it did him no favours with those Latino voters at all.

I intend to get Latinos to vote for a Republican. Mitt Romney

And even while those Puerto Rico results were rolling in, neither of the two Republican rivals were in the Caribbean territory to watch. They were both in Illinois, scene of tomorrow's crucial primary, with its broader demographic mix, and its 69 delegates at stake. Barack Obama's home state is not a natural stomping ground for Republicans, but the financial muscle power of hte Romney campaign has already blitzed the place with around $5m in advertising, while racking up an impressive roll-call of big name endorsements.

No cake walk

The latest poll from PPP does give Romney a comfortable 15 point lead, thanks to a wave of support from those describing themselves as "somewhat", rather than "very" conservative. According to PPP, Santorum's favourability ratings are slipping fast: "much worse numbers than we've seen for him most places in the lat couple months, and suggesting that GOP voters are starting to sour on him a little bit." Other polls haven't been quite so generous to Mitt, and the New York Times quotes several leading Republicans in the state, concerned that he "just doesn't get folks real excited".

Romney just doesn't get folks real excited. Jim Edgar, former Illinois governor

For his part, Rick Santorum's mostly avoided the urban heartland of Chicago and even its suburbs, spending his time and limited resources in the countryside and small towns. His single paid staffer, John Zahm, has recruited around 400 volunteers to help the effort, but organisation is so hand to mouth that Santorum almost didn't make it onto the ballot at all. He's only there in many districts because the Romney campaign decided not to challenge the fact that he didn't have enough signatures to meet the threshold.

Rural resentment

But Santorum's trying to mop up some of that rural resentment against big-city slickers, depicting a choice between "someone who can appeal to moderates and independents in New York City and Los Angeles" and, well, Rick Santorum, conservative standard bearer. "If we're going to win this election", he told the crowds, "we have to have a candidate who is going to energise our base, get them excited", before promising that if he wins Illinois tomorrow night, "I guarantee you that we will win this nomination."

Not quite the politics of class warfare, but you can detect the tinge of resentment, even among local GOP officials, like Tony Libri who told the New York Times that Romney "probably spends more on a shirt or a haircut than most Americans make in a week." It can't have helped much that Romney's latest advertising blitz accuses Santorum of "economic illiteracy", while the candidate himself dubbed his rival an "economic lightweight". This, plus having to run much further to the right than he naturally might, could put off wavering, or independent voters who are allowed to take part in Illinois' open primary.

Get with the programme

Meantime, while John McCain, who's backing Romney, has appealed to the party to "get off" polarizing issues like contraception which are in danger of alienating vast numbers of women, Rick Santorum is still determined to fight the good fight. Last week, it was single moms: now America is apparently suffering "a pandemic of harm from pornography," according to his website, which goes on to claim that the US Justice Department "seems to favour pornographers over children and families." Now there's a campaign tactic which no-one expected. Whoever's writing the Santorum playbook is at least keeping things fresh.

Felicity Spector writes about US politics for Channel 4 News.