WASHINGTON -- Democratic control of Congress may be more tenuous, especially in the long run, than post-election analysts have thus far been willing to acknowledge.

Democrats have a 233 to 202 majority in the House, but many of them picked up Republican-held seats by extremely thin margins in a very dismal environment for the GOP.

For example: Democrats gained a total of 14 House seats by less than 10,000 votes each, or roughly by a margin of 5 percentage points or less. Most, if not all, of these gains were in Republican-heavy districts that, with a more favorable political climate and good candidates, could be back in the GOP column in 2008 and likely will be.

Longer term, though, Democrats face an even bigger election problem: the ongoing population shifts from the North to the Sunbelt states that will benefit Republicans more than Democrats in the next decade and could also enlarge the GOP's electoral count in presidential elections.

While demographic and political analysts caution that Democrats have offset the GOP's Sunbelt advantage with new gains in the Northeast and have made inroads in parts of the South and Southwest, they say the sheer size of the migration by the end of this decade will help Republicans more.

"I think on balance the Republicans will benefit from the large number of seats in the Sunbelt region. They won't get 100 percent of it, but more than the Democrats do," said Merle Black, a veteran historian and analyst in southern political realignment at Emory University in Atlanta, Ga.

In a recent study by Election Data Services, which analyzes how population movements affect redistricting changes under reapportionment, the firm reported the significance of this continuing realignment. Its projections of the number of Americans moving from the Democratic Northeast to the more Republican-friendly Southern and Western states "show that seven congressional seats in 13 states have already changed at this point in the decade."

EDS forecasts that seven states -- Massachusetts, Pennsylvania, New York, Ohio, Iowa, Missouri and Louisiana -- will lose House seats and six will gain them: Florida, Georgia, Arizona, Nevada and Utah gaining one each, and Texas getting two.

But in a separate analysis, Polidata, a Washington-area demographic and political-research firm, sees a larger "probable" shift in seats from the North to the Sunbelt regions.

Under these "probable changes, there could be 13 seats shifting among 19 states, eight gainers and 11 losers. All the gainers are in the South and West and all the losers are in the East and Midwest, except Louisiana, Polidata said. The "biggest gainers" would be Texas, with four additional seats, and two each in Florida and Arizona. Georgia, Utah, Nevada, Oregon and Washington state would each gain one.

The biggest losers: New York and Ohio, losing two seats each, while Massachusetts, New Jersey, Pennsylvania, Michigan, Illinois, Minnesota, Iowa, Missouri and Louisiana would lose one.

Since each state's electoral votes are based on its representation in Congress, the shift in House seats to the Sunbelt, where Republicans are strongest in presidential elections, would mean increased clout in the Electoral College, too.

"Overall, given a 2004 electoral vote of 286 Bush to 252 Kerry, the vote count based upon these 2010 projections would have been 292 Bush, 246 Kerry, a gain of six for the Republican ticket," Polidata said.

However, analysts who have studied these projections say that while there would be GOP House gains in the South and Southwest, they note that last year's election resulted in a decline in Republican strength in the South and a Democratic increase in the Northeast.

Black points out the number of Southern Republican House seats fell from 82 to 77 seats, while Democrats saw their numbers rise from 49 to 54. "That dropped the Republican surplus in the South to 23 seats," he said.

In the Northeast, Democrats held 68 House seats and Republicans only 24, the result of a rash of GOP losses from New Hampshire to Pennsylvania -- giving Democrats a 44-seat surplus in the region.

"So for the first time in modern American politics, the Democrats' lead out of the Northeast is larger than the Republican lead in the South. If that continues, it would be a major structural change in American politics," he said. Another caveat in all this is, "We don't really know the demographics that are driving the in and out migration," says election analyst Rhodes Cook. "Some of them could be affluent white conservatives, but they might be Hispanics who tend to vote more Democratic."

Of course, redistricting can iron out the wrinkles, and Republicans will be in charge of drawing the new lines in states like Texas and Florida.

The bottom line: Democrats won in the short term in 2006 because of an unpopular war and a spate of scandals, a combination they will not have going for them in the 2012 elections. But the longer-term population trend lines are working to the GOP's advantage well into the next decade and perhaps beyond that.