One of the most common is to threaten to endorse a challenger in the next primary and back him or her with all the national party money. That is usually enough to overcome mere 'Principles,' at least in the vast majority of cases.
The deal with Ben Nelson is that no Nebraska Democrat would be suicidal enough to run against him in the Democrat primaries. He's up for re-election in 2012.
While the number of rising stars (those in their 20s, 30s, and 40s) among the Nebraska Republicans seem as unnumerable as the sands on a beach, the Nebraska Democrats have only one, the aforementioned Scott Kleeb.
Other than Kleeb, Nebraska Democrats are old fogies.
Kleeb is a descendant of Old Money and Much Money from the same place where I spent my adolescence, an ancient homesteading-and-ranching family going clear back to the 1890s. He is a graduate of Yale, and now in his late 30s.
He was in fact teaching at Yale, but then quit to come back to Nebraska, where he was a cowhand--roping cattle, mending fences, mowing hay, that sort of thing.
His first foray into politics was running for the then-open 3rd congressional district seat (the western two-thirds of Nebraska), a Democrat running against the weakest possible Republican candidate, a really really really weak candidate (I am happy to report however that during his two terms, Adrian Smith [R-Nebraska] has since been proven to have been grossly underestimated and underesteemed).
Kleeb lost that race, 45-55%, and the "narrowness" of the defeat heartened many Democrats, who conveniently forgot history. Every time the 3rd congressional seat has been "open," the race has been razor-thin, a hundred, a score, a dozen, votes--the one in 1974 was legendary. And so a defeat by ten percentage points was actually far worse than normal, when the seat is open.
Kleeb could have actually won that race--remember, he was up against the weakest possible Republican candidate--if he had taken an oath to NOT vote for Bela Pelosi for Speaker of the House, but while he's conservative, to the "right" of Nelson, for some peculiar reason he just wouldn't, and so lost.
Yale wants Kleeb back, because like franksolich, the Democrat is really into the history of the Sandhills and the Great Plains, and in fact has a degree in it. And the Institute for the Study of the Great Plains at Yale is the biggest jewel in Yale's crown.
But for now, he left the ranch in the Sandhills and teaches at Hastings College in south-central Nebraska. He's probably thinking of running for governor now, having been handily defeated in the race for the open U.S. Senate seat in 2008 by Republican Michael Johanns.
He might, or might not, have a political future, as there's nothing really negative about him, but on the other hand, he's all the Democrats in Nebraska have, to confront a well-stocked stable of Republicans, equally young, attractive, and unblemished; all the rest of the Nebraska Democrats are old farts.