An essential guide to the midterm elections

Congress has done all that it will, which isn't much, before November's elections. Which means the venue for America's permanent partisan war for now shifts exclusively to the campaign trail.

Republicans and Democrats repositioned their forces after a brief September truce for three purposes. They gave President Barack Obama authority to train and equip moderate Syrian rebels. They agreed not to shut down the Export-Import Bank for a few months. They kept the federal government open to at least December.

Speaker of the House John Boehner and President Barack Obama in 2012.
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Speaker of the House John Boehner and President Barack Obama in 2012.

None of those steps will alter the dynamics of a midterm election whose contours have been clear for a long time. Here's a quick review of what those are:

  • Fundamental conditions favor the Republicans. Most Americans believe the country is headed in the wrong direction. Average families don't see their incomes rising much even as the economy recovers. America faces a swirl of foreign policy crises. The president's party nearly always loses ground in mid-term elections, and Obama's popularity has significantly eroded. Just as the electorate in presidential years includes larger proportions of Democratic-leaning young voters, Hispanics and single women, the midterm electorate contains larger proportions of Republican-leaning whites and older voters.

  • That advantage will help John Boehner keep his gavel as speaker of the House, but his chances of padding it with a large number of new Republican seats are slim. The Cook Political Report counts only 15 House seats (12 Democrat, 3 Republican) as toss-up contests, and 26 others leaning to one party or the other. No one should expect a gain anywhere near the size Republicans amassed in their 2010 landslide.

  • Either party can still emerge with a Senate majority. Republicans, who need a net gain of six, have powerful advantages. Their candidates hold commanding positions to gain two of them—in Montana and South Dakota. Eight of 12 other critical races take place in states Mitt Romney carried two years ago. So far, their candidates have avoided the sort of crippling gaffes that led them to blow winnable Senate races in 2010 and 2012. But as they showed in those last two elections, Democrats won't be easy to dislodge. Three of their endangered incumbents—in Alaska, Arkansas and Louisiana—come from name-brand political families with multigenerational records of electoral success. In Georgia, Michelle Nunn brings the tailwind of her father Sam Nunn's popularity to her bid for an open Senate seat. Democrats have, at least for the moment, captured the high ground from Republicans in use of campaign data and technology.

  • The Obama administration's new offensive against Islamic State in Iraq and Syria represents a wildcard. Americans tend to rally around their presidents during military conflicts, as the bipartisan congressional vote to authorize the White House strategy indicates. Battlefield successes could at minimum stabilize Obama's approval ratings, and perhaps raise them a little.That would reduce a drag on Democratic prospects.

Read MoreSenate approves arming Syrian rebels

Given the gridlock that already exists, does the outcome of the battle for the Senate even matter? It does.

The midterm issues
The midterm issues   

Rare as they are, bipartisan compromises occasionally emerge from the Senate, as on immigration in 2013. Majority control influences those compromises. Control gives one party or the other a greater ability to set and shape the national political dialogue heading into the 2016 campaign. It determines whether, and how quickly, Obama's nominees to the Cabinet and the courts will get confirmation votes.

Read MoreGlobal fears give Obama a wild card: NBC-WSJ poll

The reality of contemporary politics, however, is that neither party can count on its short-term advantages for very long. Partisan control of the House has shifted twice in the last decade.

And if the Senate flips toward Republicans in 2014, it could flip right back two years later. In 2016, both the shape of the electorate and the electoral map favors Democrats. Republicans must defend seven seats in states that Obama carried twice.

And so the permanent war will rage on.

—By CNBC's John Harwood.