At the Smart Politics website, Eric Ostermeier noted yesterday that U.S. representatives from Iowa "have won 191 consecutive renomination bids from 1950 through the 2016 cycle [...] The last Iowan to lose a primary for a U.S. House seat was seven-term Republican John Gwynne in 1948. [...] The last Democrat to lose a renomination bid was freshman Sanford Kirkpatrick in 1914."
Representative Steve King beat back a more significant challenge State Senator Rick Bertrand, which wasn't surprising, given King's popularity among conservatives and support from the Iowa Republican establishment. However, the scale of King's victory (64.7 percent to 35.2 percent) was smaller than I expected. Would anyone have predicted Monica Vernon winning the first Congressional district race by a larger margin than King's over Bertand?
King carried all 39 counties in IA-04; you can view the county-level results on the Iowa Secretary of State's website. Bertrand held him below 60 percent in six counties: Cherokee (52 percent for King), Clay (56 percent), Dickinson (56.8 percent), Greene (59.4 percent), Plymouth (57.8 percent), and Woodbury (56.4 percent).
Pat Rynard saw signs of weakness for King, in that "Bertrand didn’t run a very good or well-financed campaign. If an ambitious Republican had more time and better fundraising connections, they could put together a serious challenge to King in the future."
Unfortunately, I think Iowa is stuck with King until he decides to retire from Congress. This year's primary occurred shortly after a divisive Iowa caucus campaign, in which King was a top surrogate for a polarizing figure (Ted Cruz). Yet Senator Chuck Grassley, Senator Joni Ernst, Lieutenant Governor Kim Reynolds, Secretary of Agriculture Bill Northey, and most state lawmakers in his district rallied around King.
Bertrand represents much of Sioux City (the largest metro area in IA-04) in the Iowa Senate and owns a successful business there, but he still couldn't manage more than half the Republican votes in Woodbury County. I heard about GOTV calls to Democrats in the fourth district, urging them to change their party registration so they could vote against King, but Bertrand didn't do particularly well in Democratic-leaning areas like Story County (Ames) or Cerro Gordo (Mason City).
I have trouble imagining any candidate building a case that could either convince Republicans to reject King or turn out enough Democrats and independents to win a GOP primary. Successful primary challenges to Congressional incumbents in either party, anywhere, are few and far between. The advantages of incumbency in terms of name recognition, money, and support from powerful interest groups are too much to overcome, barring a major scandal or other extraordinary circumstances.