The first night of the Republican convention last week was not subtle in its focus. Multiple speakers addressed the 2012 terror attacks in Benghazi, in keeping with the evening's theme: "Make America Safe Again." The focus on national security and law-and-order was relentless, save for the keynote speech from Melania Trump.

The first night of the Democratic convention was different. The focus wasn't on national security, it was on party unity. But it didn't take Republicans long to notice that mentions of security weren't just rare, they were mostly absent -- and that mentions of the Islamic State were non-existent. Politifact evaluated a party press release claiming that the subject was never mentioned. Rating: True.

During a brief speech at the annual Veterans of Foreign Wars convention in North Carolina, Donald Trump attacked his opponents for ignoring the subject.

"We need to change our foreign policy to focus on defeating and destroying ISIS, a word you didnt hear last night at the Democratic convention," he said, using another term for the group. "You didn't hear it. They dont want to talk about it. Because in a very true way they really established ISIS, because of weakness." (That "very true" claim has also been evaluated. Rating: False.)

Given that this was day one of four, the Democrats have plenty more opportunities to raise the subject of national security. What's more, the critique is an old one in politics, employed by both sides: "Why didn't they talk about [issue X]? That's the real issue in this campaign!" But there's a good reason for Republicans to focus specifically on this issue in particular.

If you ask voters what issues are most important to them this year, there's one answer that's most common: The economy. About 1-in-5 voters gave that response to Gallup in June. When Quinnipiac University asked later in the month, it was twice as common a response as the next-most-common answer, immigration. In Gallup's poll, only 5 percent of respondents cited terrorism as their main concern; in Quinnipiac's, it was 3 percent.

But just because it's not voters' top concern doesn't mean it's not a concern. Nearly 9-in-10 voters told Gallup terrorism and national security was a priority. When the firm asked the same question in February, both Republicans and Democrats said it was near the top of their concerns.

Notice that the Republicans (and Trump) didn't mention foreign policy broadly, though -- they mentioned the Islamic State. In Washington Post/ABC News polling, there's a big difference in who voters prefer on national security broadly and terrorism specifically. Hillary Clinton and Trump run a lot closer when voters are asked who they trust to handle "terrorism" versus "an international crisis."

That gulf is more dramatic when you look at individual states. Quinnipiac surveyed voters in the swing states of Florida, Ohio and Pennsylvania, finding that Clinton was preferred to Trump on the question of handling an international crisis. But on the specific question of the Islamic State, Trump fares much better, particularly in Florida.

(Notice, too, Trump's advantage on jobs.)

What the Republicans are doing in calling attention to the Islamic State is to reinforce to voters that it is a priority for them, allowing Trump to benefit further from his advantage on the subject. The Islamic State offers another political advantage, grim as it is to say it. The regularity of attacks increases the odds that one will occur in the vacuum before the Democrats mention it, as happened in France Tuesday morning.

This is why campaigns employ the "why aren't they talking about [issue X]" tactic: Even when the Democrats do focus on the issue, the Republicans have spent 24 hours making hay from their decision to skip it in the first place.