If you scratch any Republican pundit on the face of the earth, a comparison between Trump's 2016 campaign and Rudy Giuliani's 2008 campaign will fall out. If the point is to illustrate that being the frontrunner at this stage of the race won't necessarily translate to victory, then these pundits are on solid ground. If the point is to draw a meaningful parallel between the current Trump boom and the forces that drove Giuliani to the top, and then back down to the bottom, of the 2008 pile, then they are dead wrong.
By far the more meaningful comparison to Trump 2016 is the Howard Dean 2004 campaign.
Howard Dean's improbable 2004 run, which was viewed with horror by the Democrat establishment of the time, was motivated by almost the exact same forces that are currently animating Trump's run. Democrats in late 2003 and early 2004 were, beyond anything, angry at their party's establishment for their perceived (and actual) unwillingness to stop or even slow the agenda of George W. Bush. Therefore, they ignored their party establishment's urgent warnings that Dean was too hard-edged, too frank, and too willing to engage in unseemly insults against his opponents.
To Democrat activists in 2004, Dean's personality foibles were the raison d'etre of the Howard Dean candidacy. Features, if you will, not bugs. And Democrat primary voters allowed themselves, at least for a period of several months, to not care about the mounting evidence that Dean would be a dream general election opponent for the hated George W. Bush.
The first sign that things were amiss in the Dean campaign came not in his national poll numbers, but rather in Iowa, where he was perceived to have previously run with strength. Polls taken just days before the caucuses actually forecast that Dean would lose to both Kerry and Edwards (even though Dean had been running in first there for most of the campaign), but for Dean, the key was to remain ahead of Gephardt for third, which he successfully did. However, in the course of turning his fiery rhetoric on Gephardt, Dean alienated himself from liberals both in Iowa and nationwide, and as a result finished a surprisingly anemic third.
Dean should have still been in a strong position going forward, since New Hampshire was right next door to his home state of Vermont, and national polls still showed him in the lead. However, Dean's ugly personal meltdown in Iowa, culminating with the yeargh heard 'round the world, ultimately doomed him in New Hampshire, and Kerry was off to the races.
What killed Howard Dean was that he was no more able to take crap from Democrat voters - or other Democrat candidates - than he was from George W. Bush. There's a way that you can lose Iowa in devastating fashion and still survive to win the nomination, as numerous candidates from both parties have done before. If you can display grace in defeat, present a cogent plan for your path to the nomination without Iowa, and show that you have learned from the experience, losing in Iowa doesn't even have to be that damaging.
However, nothing thus far has indicated that Donald Trump has the capacity to demonstrate any of those things. His increasingly panicked response to the threat [mc_name name='Sen. Ted Cruz (R-TX)' chamber='senate' mcid='C001098' ] presents to his chances in Iowa - even before the first votes are cast - suggests that taking a strategic loss (in politics) is not something Trump is prepared to handle on a personal level.
It may, in a parallel to 2004, cause Trump to go increasingly negative against Cruz in the same way Dean went negative after Gephardt - which may well cause Limbaugh, Levin, et al to turn on Trump for good. Whether Trump can retain his lead without those two carrying his water day after day (or worse, with them actively fighting against him) is perhaps a question that Trump doesn't want the answer to.
But if the polling is any indicator, Trump ought to prepare for losing Iowa as an inevitability rather than as a possibility, especially given that Cruz has built a maniacally efficient ground game and targeting system in Iowa that will likely lead to him outperforming his standing in the polls. If he loses it like Mitt Romney did in 2012, he will be fine. If he loses it like Howard Dean did in 2004, he will be toast.