Looking back to 2008 it looks like Mitt Romney has been running for President ever since he was in the 2008 Republican primaries where he lost to John McCain. Considering a short stoppage while McCain was the Republican candidate, and supposing that Mitt Romney started his campaign again soon after Obama won, I know it was soon after but don't have the date, that would make him one of the longest running candidates - about 4 years.

Are there any other cases where a Presidential campaign in the US has gone on as long, or longer, or is about 4 years the longest on record? This should be an individual running non-stop within a single political party, in an attempt to be their nominee for President.