Jim Cole/AP

Foreign policy and the Middle East as a major issue in the midterms? A few months ago, it would have been almost laughable—but with overseas news dominating the headlines and on voters' minds, Republicans see the issue as the final piece in the puzzle for using fears about President Obama's tenure against Democratic candidates in key races.

In the battle for Senate control in November, Republican strategists say GOP candidates can use foreign policy to help tell a story about Obama—and, by extension, Democratic Senate candidates—as incompetent and providing insufficient leadership for the country.

"If you'd asked us two or three months ago what kind of role is foreign affairs going to play in this election, nobody would have said it was going to play a role," said GOP pollster Neil Newhouse. "But now you see current events taking over and impacting voters' concerns—it's current events that have forced this issue onto the front burner."

Foreign policy was never expected to be a top issue in this year's midterms, which thus far has been dominated by health care and economic issues. It's typically not something voters want to hear much about even in a presidential race, let alone the midterms, and candidates tend to shy away from using fast-moving current events in TV advertising.

But voters are really paying attention—and their opinions on the issue aren't good for the president or for his party. Obama's big ISIS speech Wednesday night came just a day after an NBC News/Wall Street Journal poll found his approval rating on foreign policy had plummeted to 32 percent. Voters gave Republicans an 18-point advantage on the issue of foreign policy, up 11 points from this time last year. And a whopping 94 percent of those surveyed said they had seen or heard about the beheadings of journalists James Foley and Steven Sotloff—more than any other news event NBC/WSJ had tested in the last five years.

"The developments of the last month, particularly the ISIS developments, have really permeated the frontal lobe of the American political consciousness," said GOP strategist Phil Musser. "And so when issues, whether they're domestic or international, break through in that kind of way in the final throes of an election year—inevitably they become part of the narrative."

The GOP argument goes something like this: The summer's foreign-policy headlines, with the rise of ISIS in the Middle East and Russia's encroaching power in Ukraine, are—coupled with economic concerns, the border crisis, and Obamacare—further proof of Obama's incompetence as chief executive. Democratic candidates in key races, particularly incumbents who've frequently voted with Obama, will support his initiatives overseas. Therefore, voters who want to change the course of the country's leadership should vote against Democratic candidates this fall.

That was the central theme of two recent ads from Senate Minority Leader Mitch McConnell, who's facing off against Democrat Alison Lundergan Grimes in Kentucky.

"These are serious times," a narrator says in one, as news footage of ISIS, national unemployment, and the border crisis flash across the screen. "In Kentucky, we have a proven leader—when somebody in Washington can't do the job, shouldn't Kentucky have a senator who can?"

Elizabeth Wilner of Kantar Media, which tracks political advertising across the country, said that the number of TV spots featuring foreign policy or ISIS has been fairly small thus far but is expected to grow.

"You certainly can't call it a flood, but we're starting to see a trickle—it's likely, particularly after the president speaks, we're going to see more," she said. "It certainly does look like a broad line of attack Republicans are going to start using against Democrats for the next two months."

Democrats' responses to the situation have been varied. Some incumbents with tough reelection battles, like Senator Kay Hagan of North Carolina and Senator Jeanne Shaheen of New Hampshire, have criticized the president's handling of the situation; others have trod carefully on the issue, avoiding spending too much time talking about Obama's actions.

The National Republican Senatorial Committee has been playing up the importance of foreign policy, sending out a memo to reporters Wednesday with polling proving the importance of foreign policy this fall.

"Is foreign policy going to stay as hot as it is right now? I don't know," NRSC Executive Director Rob Collins said Tuesday at an event at the National Press Club. "But we have to prepare for it."