US Republican presidential hopeful Rick Santorum has decided to 'suspend' his campaign.  Al Jazeera's Alan Fisher who has been following the Republicans on the campaign trail looks at what the decision means for the former senator and for the front runner, Mitt Romney.

Why has Rick Santorum stepped out of the race?

There are a number of reasons. The main one is he can't win. Mitt Romney is so far in front in the all important delegate count, he can't overhaul him.

The Republican Party would like to see an end to this contest to allow them to concentrate money and resources on the big battle in November.

Added to that is the opinion polls which suggest that Santorum may lose the primary in his home state of Pennsylvania at the end of the month. That would be hugely embarrassing politically. Remember, he was voted out of his senate seat there. To lose a second vote on your home turf is tough to explain away.

And then there is his young daughter's illness. She is severely disabled and has been hospitalised twice in recent months. Whatever critics think of Rick Santorum they accept he is a devoted family man and so that was a not insignificant element.

Is Santorum a possible pick for Vice President?

Making a prediction in this highly unusual contest is a bit of a gamble but I think there is no chance of that. Santorum would find it difficult to go from saying Romney would be the absolute worst Republican candidate to debate President Obama on his signature healthcare policy to hailing him as ideally placed to be the leader of the free world. That would take extraordinary political gymnastics and would be punished in adverts and campaigning by the Democrats.

Santorum says the fight's not over. What does that mean?

He has pursued a social conservative right wing agenda through the nomination process and it has struck a chord with many in the party. He does not want many of the issues he has raised to disappear - so he'll keep making speeches and make sure Mitt Romney continues to support many of the right wing positions he's been forced to adopt during the campaign. He’ll try to keep the nominee ‘honest’.

Four months ago, many voters would have struggled to identify the former Pennsylvania senator. He has now emerged as a leading conservative voice in the party.

Will he run again?

He will undoubtedly have learned lessons from this experience. He was poorly funded, his campaign was poorly organised and he made important strategic mistakes.

If Mitt Romney wins the White House, it would be eight years before he could make another run. If Obama wins, it is would be only four years but the Republican Party was deeply unimpressed by the candidates on offer this time around, and so he could be superseded by the likes of New Jersey governor Chris Christie or Louisiana Governor, Bobby Jindal or even former Florida governor Jeb Bush who opted out this time around. This may have been his best chance.

Will Mitt Romney be celebrating?

Baring some unforeseen disaster - Santorum's departure means he will be the Republican who'll take on Obama in November. However he has a problem. Fifty nine percent of those who've voted in the nomination process have voted for other candidates. He has to try to rally Santorum supporters and the conservative core - which he has failed to do so far, but do that without alienating the moderates and independents he needs to win. He has to keep his right wing happy while moving to the political centre. That isn't easy.

Will Obama be happy with Santorum's departure?

Even though most polls show he has a lead over Romney (he had a bigger lead over Santorum) the White House knows it's in for a battle. The economy remains the issue and the recovery is slow and painful.  Add the high petrol prices the Obama administration needs some good economic news well before November. However, he finally knows who he will face. And that means the US Presidential election begins now.