1. If Romney loses, and if you want to try and designate the moment he lost it, I think there are two candidates. One is when he penned the editorial "Let Detroit Go Bankrupt." True, he didn't choose the title, but he didn't protest it too much either. It is one of the few positions he took in the gulf between the 2008 campaign and the 2012 one and it turned out it was both wrong and politically damaging. He might have won Ohio without it. The other moment was when he overestimated the threat that Perry represented and tacked to the right of him. This caused him to go hard on the issue of the immigration. I think a Mitt Romney who supported the Dream Act would be the favorite today instead of the underdog. But that Romney was strangled to beat a candidate who could have just been ignored.
2. Romney has been out of elected office for 6 years. The last candidate from one of the two major parties to have been out of office that long was Dwight Eisenhower in 1952, because he never held elected office before being elected president (making him something of anomaly). If we don't include him for this reason, we have to go back to Wendell Wilkie in 1940 - who also never held elected office. Then there's Hoover in 1928. But if we want someone who once held elective office, but had been out for 6 years or more, you have to go back to John W. Davis in 1924. By the time he ran for president, he'd been out of the House for 11 years (though during that time he'd been Ambassodor to the UK and Solicitor General). Let's say we want to find someone who never held an appointed or elected office for at least 6 years. That eliminates Eisenhower and Hoover (though Wilkie still never served in the government).
So, what if we want someone who once served in elected office, but then didn't serve any government role for 6 years and was the nominee of one of the two major parties of the time. How far do we have to go back then? We have to go back to William Jennings Bryan who in 1908 had been out of the House for 13 years.