Knew it all along. Portman was never going to run. With Kasich's strong grip on the Ohio GOP, he probably convinced Portman to stay out of the Presidential (so Kasich could pursue his own run) in return for the state party's establishment support of Portman for re-election. That and Portman would be hopeless in a primary. His support for gay rights, immigration reform, his willingness to compromise with Democrats, and his extreme hatred by the Tea Party all do him in.
Portman will certainly face a few right-wing challengers in his re-election bid, but to a lesser extent and for different reasons. The far-right in the state are upset with him not focusing enough on Ohio and his out-of-state fundraising for other Republicans, but he can easily survive a state primary fight. Strickland and Ryan will definitely be discouraged to run now and most other Democrats in our field just aren't ready to run statewide yet or may not be strong enough to beat Portman. Nonetheless, Portman made the right decision for his political future. He'll be in the minority caucus again come 2017, but he has a much better chance at retaining his Senate seat now. 
Why would he be in the minority caucus come 2017?
There is no guarantee that the Democrats will win back the Senate and the best odds of that happening are about 50%.
However, Portman not running indicates that there is going to be a coordinated effort by the establishment candidates to have one establishment candidate remaining before South Carolina, so that he could win that primary and effectively seal the deal on Super Tuesday.
