CQ Politics’ color-coded map of the 2008 presidential results in each of the 435 congressional districts doesn’t at first glance reflect Democrat Barack Obama ’s dominant showing. It seems to be awash more in “Republican red” than in “Democratic blue.”

Obama decisively won the election, of course. So why does the map depicting the district results seem to have a disproportionately crimson coloration?

It’s because Obama’s strongest showings were in populous urban and inner suburban areas — which are more geographically compact than the outer suburban and sprawling rural areas where Republican presidential nominee John McCain ran well.

Obama defeated McCain in 92 of the 100 congressional districts with the smallest land areas, according to a CQ Politics analysis that matched the election results with Census Bureau data on the square mileage of congressional districts. Democratic candidates for the U.S. House also cleaned up in these high-density districts, winning 87 of 100.

The more wide-open spacious a congressional district, though, the better McCain did. McCain defeated Obama in 73 of the nation’s 100 largest districts by land area. These areas also tend to be less racially and ethnically diverse and more conservative on cultural issues than the nation as a whole, and are virtual polar opposites of the tightly compact inner-city districts where Obama ran up the score.

The red palette of the presidential vote map is influenced by the fact that McCain carried five geographically huge but sparsely populated Western states — Alaska, Montana, North Dakota, South Dakota and Wyoming — as well as both districts in Idaho and rambling rural districts in places such as eastern Oregon, northern Nevada and western Nebraska.

Yet, symbolic of the trends in congressional politics over the past two election cycles, House Democrats are more than holding their own in these roomy areas. They hold 47 of the 100 largest districts, or 20 more than Obama won. That’s greater than the difference between the total number of districts that House Democrats won (257) and those Obama won (242) in the 2008 election.

Some freshman Democrats who won last year to take over large districts that had been held by Republicans will be put to the test in the 2010 elections. Betsy Markey , who represents northern and eastern Colorado, and Walt Minnick , who represents western Idaho, almost certainly will face serious opposition next year in districts that voted for McCain. So might Harry Teague , who represents a southern New Mexico district that is as large as Missouri, and Ann Kirkpatrick , elected last year to a northeastern Arizona district that is comparable in size to Georgia.

But other Democrats from rural districts have settled into political security. The largest represented by a Democrat is the at-large district in South Dakota (75,885 square miles), which has a single House member. Stephanie Herseth Sandlin was easily re-elected there in 2006 and 2008 after winning her first two elections by much narrower margins.

Democrats Earl Pomeroy of the at-large district in neighboring North Dakota and John Salazar , who represents western and southern Colorado, also won easily in 2006 and 2008.

At the other end of the size spectrum, the urban districts dominated by Obama and, in nearly all cases, House Democrats often have large percentages of racial and ethnic minorities who have long made up the foundation of the party’s voting base — and who turned out in especially large numbers in 2008 to support Obama in his successful effort to become history’s first black president.

The two smallest districts in the nation — New York’s 15th, represented by Democrat Charles B. Rangel , and New York’s 16th, represented by Democrat José E. Serrano — also were the strongest pro-Obama districts in the 2008 election. Obama took 95 percent in Serrano’s district, based in the south Bronx, which is Hispanic-majority, and 93 percent in Rangel’s Manhattan district, which includes the historic African-American area of Harlem and where Hispanics are a large plurality of residents.

The six next-smallest districts are also wholly based within New York City, and all are liberal bastions that voted overwhelmingly for Obama and for House Democrats: the 11th, in Brooklyn ( Yvette D. Clarke ); the 14th, which takes in parts of Manhattan and Queens ( Carolyn B. Maloney ); the 8th, a Manhattan-Brooklyn district ( Jerrold Nadler ); the 10th, in Brooklyn ( Edolphus Towns ); the 12th, with chunks of Manhattan, Brooklyn and Queens ( Nydia M. Velázquez ); and the 7th, with parts of Queens and the Bronx ( Joseph Crowley ).