The men hoping to face Barack Obama, the US president, in the upcoming US general election have faced another critical test with Michigan and Arizona choosing their Republican candidate.
In the end Mitt Romney, the former Massachusetts governor, won both. But Michigan, a state with which he had strong personal ties - he grew up there and his father was a popular governor - became a much tighter contest than might have been expected.
"Romney is a tragically inept candidate. He is incredibly out of touch with the base voters. The Republican Party has a great, big problem. He has the money to force his way through but he doesn't have the personal appeal to ever close the deal."
- John Nichols from The Nation magazine
He only managed to beat his closest rival, Rick Santorum, the former US senator, by three per cent, leading him to declare: "We didn't win by a lot, but we won by enough and that's all that counts."
But Santorum has proved very popular with those who are among the most right-wing of the Republican Party.
And he has not shied away from making provocative comments, declaring at various stages of the campaign:
"President Obama said he wants everybody in America to go to college. What a snob." "We have a president who is uniquely weak in the face of evil." And: "You hear so much about separation of church and state. I'm for separation of church and state. The state has no business telling what the church to do [sic]."
The other two candidates, Newt Gingrich, the former House of Representatives speaker, and Ron Paul, a congressman, have been focusing on the 10 states up for grabs next week on what is called Super Tuesday.
The Republicans are competing for an overall majority of delegates. These are the party members with the power to vote at their convention in August.
"Santorum would have walked away with a victory had he not decided to engage in a war against college and Kennedy. Both of those positions made him seem like he was totally out of the mainstream, somebody who could not go against Obama and conceivably win. That's his weakest point."
- Faiz Shakir, a Democrat and Obama supporter
The magic number of delegates needed to win the Republican nomination is 1,144. The current tally, according to Al Jazeera's broadcast partner NBC news, shows Romney leading with 122 delegates after 11 contests - twice as many as the rest of the candidates combined.
Meanwhile, Obama has also been on the campaign trail.
Addressing the United Auto Workers union, one of the largest unions in Michigan on Tuesday, he poked fun at Republicans who criticised the government bailout of the motor industry, saying:
"It's been funny to watch some of these folks try to rewrite history. The same folks who said if you bail out Detroit you can kiss the American auto industry goodbye. Now they're saying 'we've been right all along'."
So, following another bruising campaign, and one which descended into bitterness with Romney accusing Santorum of "dirty tricks", the question yet again is: Why has it become so difficult for Romney to close the deal? Does he have what it takes to clinch his party's nomination or to take on Barack Obama for the White House?
What do these latest Republican battles tell us about the remaining candidates, their ability to connect with voters and how they might match up against Obama in the general election?
Joining Inside Story: US 2012 with presenter Anand Naidoo to discuss this are: John Nichols from The Nation magazine; Ford O'Connell, a Republican strategist and former adviser to the 2008 Republican campaign; and Faiz Shakir who worked for the Democratic National Committee and is an editor of the political blog ThinkProgress.org.
"What America is looking for right now though is somebody who is a CEO, not somebody who sits in government life. One of the big problems that Obama has in trying to get the economy back on track is that he's not looking at it from a pro-growth strategy. What Romney is saying about the tax code is far better in pushing America forward."
Ford O'Connell, a Republican strategist