For a long time there was a simple formula that worked quite well for Republicans: say
"We're strong, they're weak," advocate force in foreign affairs as often as possible, repeat, win elections. But ten years since the Afghanistan war started and just under nine years since the Iraq war started, there's no obvious political benefit to taking the position that what we need is another war. When you combine that with the series of badass special operations missions (most notably the killing of Osama Bin Laden) that have occurred under Obama, it's not obvious that advocating a war with Iran makes you look tough. It may just make you look like a nut, or an idiot.