As the midterm election enters its last six weeks, new factors have appeared that will help determine which party controls the Senate next year.
Surprisingly, national security has emerged as an important issue. Americans' concerns began with the conflict in Ukraine, and they've grown as Islamic State cut a swath across Iraq and beheaded two American journalists.
When an Aug. 4 CBS News poll asked what was the most important problem facing the country, terrorism wasn't even on the list. In a Sept. 15 CBS News/New York Times survey, voters ranked terrorism as No. 2 at 17% (behind the economy at 38%) as the most important factor in their vote for Congress.
A foreign-policy crisis normally boosts the party in power. Not this time. Mr. Obama's abysmal approval numbers are likely to improve only a little. The GOP's advantage as the party better able to handle national security has been building for more than a year and is now 23 points in the Sept. 7 Gallup poll. This advantage will not easily be reversed. By taking America into a prolonged war in the Middle East Mr. Obama will also depress his party's large peacenik wing.
Still, the economy remains the top issue for the electorate. That's why Mr. Obama awkwardly tacked onto his Sept. 10 prime-time address on the Islamic State several paragraphs heralding America's economic health.
The Democratic Party's problem is that voters don't believe the president's claims that the economy is thriving. Even people with jobs feel apprehensive. Paychecks are flat, growth anemic, and people are worried about their children's prospects. Mr. Obama had a 38% approval on handling the economy in the Sept. 9 Fox News poll. In the Sept. 7 NBC News/Wall Street Journal poll, 67% believe America is on the wrong track.
The economy will affect the election differently than in 2010. Then Democrats held all Washington's levers of power. To win, most Republicans simply had to attack Democrats for supporting the stimulus and ObamaCare.
Four years of divided government with the GOP controlling the House has left some conservatives and independents disappointed. They expected the Republican House would somehow dominate and direct the Democratic Senate and White House. That was a pipe dream. To win in November, Republican candidates must show they are in touch with voter concerns about growth, jobs, paychecks, government spending and debt. The only way to do this is by offering specific, persuasive ideas.
Immigration once provided Democrats an edge. This summer's wave of Central American children crossing the southern border nullified that advantage. Mr. Obama's threat to grant amnesty to millions of illegal immigrants by fiat helped turn immigration into an issue involving the president's abuse of power, not just an issue of national security and the economy.
Already Democrats are turning from their "war on women" meme to their more traditional scare tactic of arguing that Republicans will gut Medicare and Social Security. They may close out their campaigns by demonizing Senate Minority Leader Mitch McConnell or, more likely, arguing every Republican candidate will be an extremist committed to gridlock, hyper-partisanship and shutting down the government.
How successful this will be depends on how strong an impression voters have of what Republicans will do if they run the Senate. It is not enough for Republicans to remind them of Mr. Obama's many failings; voters want to know how the GOP would move the country forward. Republicans must keep three plates spinning at once: encouraging voters to send Mr. Obama a message, defending themselves against brutal Democratic attacks, and laying out a governing vision.
We will soon start to get a feel for the effectiveness of each party's get-out-the-vote efforts. Democrats have ready-made GOTV organizations in the 2012 presidential battlegrounds of Colorado, Iowa, North Carolina and Virginia, but not in any other states with Senate races. The Republican National Committee and GOP campaign committees have emphasized the ground game more this year than in any election since 2004.
Absentee and early voting have begun in some states. Each party's performance in getting out its early voters, compared with 2010 and 2012, could foreshadow November's results.
The impact of any factor may be small in any given race. But in tight contests, they can be decisive. Control of the Senate and the direction of President Obama's last two years in office may depend on it.
A version of this article appeared September 25, 2014, in the U.S. edition of The Wall Street Journal, with the headline The Issues Are Breaking Bad For Democrats and online at WSJ.com.