As Justin Sink reports in The Hill, President Obama’s job approval rating has been dropping significantly in the heavily Democratic states of New York and California.

Ordinarily, that wouldn’t be of much interest in a campaign year, since Democratic statewide candidates would be likely to win in any case, and incumbent Governors Andrew Cuomo and Jerry Brown (interestingly, both sons of former governors) are polling well ahead of their Republican opponents, though only at slightly more than 50 percent themselves.

But the New York and California numbers are important in House races, because a disproportionate number of close House races are in those two states.

The latest Cook Political Report House race ratings, updated last Friday, lists 68 House races as being in play — 14 likely Democratic, 13 leaning Democratic, 11 Democratic toss-ups, four Republican toss-ups, eight leaning Republican and 18 likely Republican. Only one currently held Republican district is listed as leaning Democratic (California-31, where under the state’s current system two Republicans faced off in the 2012 general election, guaranteeing a Republican victory in a 57 percent Obama district), and only two currently held Democratic districts are listed as likely Republican ( North Carolina-7 and Utah-4, where Democratic incumbents barely won in 2012 in heavily Republican districts and both are retiring this year).

Let’s leave those districts aside and consider the other 65 districts, 38 currently held by Democrats (including New York-21, an open seat listed as leaning Republican) and 27 currently Republican districts. Of the 38 Democratic districts, 11 are in New York and California, and another eight more are in Illinois and Arizona.

All four of those states had Democratic redistricting plans: New York, because it was hard to draw many Republican districts; Illinois, because of Democratic control; and California and Arizona, because Democrats successfully gamed supposedly nonpartisan redistricting commissions. The only other state with a similarly Democratic-leaning redistricting was Maryland, where no seats are seriously contested this year.

What this means is that of the 38 Democratic districts in play, exactly half — 19 — are in states with Democratic districting plans and in states where either Obama’s job approval has been dipping (New York and California), where it was never much above 50 percent (Arizona) or where the Democratic governor has been running behind in most polls and the Obama home state advantage has probably dissipated (Illinois).

This is more or less what we should expect: Partisan redistricters tend to be optimists and thus create many districts where their party is ahead in years when opinion is favorable to the party, but may dip below 50 percent in years when it’s unfavorable. In the heavily Democratic years of 2006 and 2008, Republicans lost many such seats in states with Republican redistricting plans, including Pennsylvania, Michigan, Wisconsin and Florida.

In addition, seven of the 27 Republican seats on Cook’s list are in those four states.

The upshot: Declining Obama job approval in New York, California and (probably) Illinois, plus a negative Obama environment in Arizona, put a little wind behind Republicans in House races. And remember that in 2012, when his job approval was 50 percent, Obama carried only 209 House districts, while Mitt Romney carried 226.

The shape of the political landscape suggests Republicans will gain House seats from the 234 they won in 2012, and possibly exceed the 242 seats they won in 2010 — more than the party has won in any election since 1946.