I would say that in 1952 (Robert Taft), 1964 (Nelson Rockefeller), 1968 (Rockefeller, George Romney), 1976 (Ronald Reagan), 1988 (Robert Dole), 1996 (Steve Forbes), 2000 (John McCain), and 2008 (Mitt Romney), the nominations were up for grabs.
It all depends on what one means by "up for grabs". In every single presidential nominating contest for both parties from at least the last four decades, the frontrunning candidate has not simply had a smooth ride to the nom., with no major challenges along the way. (Well, OK, not *every* time. I'm not counting cases where you have an incumbent president with no opposition or token opposition.)
So, for example, in 1996 and 2000, both Dole and Bush ran into rocky waters for a while there, where their nomination was in doubt. But was there ever actually a moment when when they *weren't* the frontrunner? I'd argue, probably not. After NH, for example, people were saying that McCain had a chance, but virtually everyone still regarded Bush as the frontrunner.
