Only 11 of the 25 Senate races have contested general elections at the moment. All are bunched in the partisan middle, with the top seven Democratic seats that are on this cycle unopposed, and the top six Republican seats also going uncontested. (This is using my oversimplified math of active registration, Ds minus Rs, which is just an easy approximation.)
The toughest races: Whoever comes out of the Senate 29 Republican primary to face Tod Bowman has to run in the #12 Democratic district overall (counting the holdovers, which are 12 Ds and 13 Rs). Democrat Maria Bribriesco is challenging Roby Smith in the #17 Republican seat. The only seat that's uncontested within that range is Matt McCoy's, the #13 Democratic seat.
The only Senate or House race with primaries on both sides is Senate 39, the #23 Democratic/#28 Republican seat. Sandy Greiner is retiring, but never ran in this version if the district which is half in Johnson County.
Moving over to the House:
At the moment there are 58 uncontested general election races. 26 seats have no Democrat and 32 no Republican.
Republicans have candidates in 68 seats, with 63 of those in their top 65 seats. The most vulnerable Democrats with byes for now (there can still be conventions till August) are Patti Ruff in House 56, their #26 seat, and John Forbes in House 40, their #37 target.
The GOP's toughest races for now are: Nathan Bolton in House 61 in Waterloo. That's the #80 Republican seat, but Republicans actually won it in 2006. The second toughest GOP seat, #70 on their depth chart, is one of their own: incumbent Brian Moore in House 58.
There are four seats with Republican primaries and no Democratic candidate: open House 1 and primary challenges for Jake Highfill, Dave Heaton and Stan Gustafson.
Democrats have candidates in 74 seats, including all of their top 46 seats and three gaps in the top 65. The uncontested Republicans are tougher targets than the unopposed Dems: Tom Sands at #47, Kevin Koester at #50 and Larry Sheets at #51. Note that even after the August nominating convention deadline, Pat Murphy had only recruited 70 candidates.
There are three Democratic primaries: the open Pat Murphy and Tyler Olson seats and an odd primary challenge to Vicki Lensing in Iowa City.
The Democrat with the toughest uphill fight is Tim Ennis challenging Jack Drake in House 20, the #10 GOP seat.
Democrats are challenging 22 Republican incumbents, while the Republicans are taking on just 13 Democratic incumbents.
And finally, the lucky duck of the year is former Republican senator Mike Sexton. He looks likely to walk into Tom Shaw's House 10 with no primary or general election opponent. Still not as good as Nancy Dunkel last cycle: she walked into a seat held by the other party with no opposition.