"What we're really seeing here are the last vestiges of a Reagan/HWBush-era Republican Party that took foreign policy seriously on its merits."
What is missing, specifically? The Republican side of "establishment" foreign policy. That is, a group of people who are certainly Republicans, but are not particularly partisan and who are comfortable working with the similar set of Democrats. Think Dick Lugar; think Colin Powell; think, perhaps more than anyone over the last 50 years, George H.W. Bush. Those Republicans, as Lugar's defeat for re-election last year demonstrated, have been driven to the fringes of their party (or perhaps out of it; Powell is still a Republican, but supported Barack Obama in 2008 and 2012). Why does that matter for Barack Obama? There just are not very many Republicans remaining who both care about foreign policy and national security and who are also inclined to work with a Democratic president as a matter of course. Those who do have virtually no clout within their party. Which means that when Obama proposes something, he starts with essentially the same zero Republican votes that he starts with on domestic-policy proposals.