WASHINGTON — Even if Democrats recruit great candidates, raise gobs of money and run smart campaigns, they face an uphill fight to retake control of the House in this year’s congressional elections, regardless of the political climate in November.

The reason? Republican strategists spent years developing a plan to take advantage of the 2010 census, first by winning key state legislatures and then redrawing House districts to tilt the playing field in their favor.

In states like Ohio, Michigan and North Carolina, Republicans were able to shape congressional maps to pack as many Democratic voters as possible into the fewest House districts. The process, called gerrymandering, left fertile ground elsewhere in each state to spread Republican voters among more districts, increasing the GOP’s chances of winning more seats.

Geography helped, too, in some states. Democratic voters are more likely to live in densely populated urban areas, making it easier to pack them into fewer districts.

The first payoff came in 2012, when Republicans kept control of the House despite Democratic support that swept President Barack Obama to a second term. The next payoff is likely to come this fall.

Gerrymandering has a long history in the United States, pursued enthusiastically by both Democrats and Republicans. But the GOP’s success at it this decade has been historic: In 2012, Republicans achieved a 33-seat majority in the House, even though GOP candidates as a group got 1.4 million fewer votes than their Democratic opponents.

It was only the second time since World War II that the party receiving the most votes failed to win a majority of House seats, according to statistics compiled by the House Clerk. Democrats gained eight seats but were still a minority.

“The fact that Republicans controlled redistricting (after 2010) meant that they were able to build up a wall, stopping a lot of the tide from running out,” said Justin Levitt, a law professor and redistricting expert at Loyola Law School in Los Angeles. “They were able to shore up a lot of the districts that had been won by, in many cases, tea party freshmen or other Republican freshmen.”

The Republicans’ advantage will fade as the decade wears on and the population changes. In the meantime, Democrats control the White House and the Senate, while Republicans control the House, giving the GOP powerful leverage to block Obama’s second-term agenda.

How did Republicans gain their advantage? It all started with the party’s sweeping victories in 2010 and a plan called REDMAP.

The 2010 election was a disaster for Democrats. Republicans picked up 63 seats to win control of the House. They also gained seats in the Senate, though Democrats kept a majority.

Perhaps more important, Republicans won control of state legislatures in crucial states, giving the party the edge that is still paying dividends.

Every 10 years following the national census, states redraw the boundaries of House districts to account for population changes. Some states gain seats and others lose, so the overall total remains 435. In most states, the legislature and the governor draw up the new districts, which is why political parties pay special attention to elections at the start of each decade.

“I think Democrats made a terrible mistake. They did not put nearly enough attention or resources into legislative races at the state level,” said Matt Bennett, a former aide to President Bill Clinton. “A bunch of these legislatures slipped by very narrow margins, and some of them flipped for the first time since Reconstruction in the South.”

For Republicans, it was a combination of luck and planning. The political winds were in their favor, but they also had been plotting for years to take full advantage of redistricting.

REDMAP, which stands for Redistricting Majority Project, called for targeting races in states that were expected to gain or lose congressional seats. GOP strategists reasoned that redistricting could have a greater impact in these states because there would have to be more changes to district boundaries, said Chris Jankowski, former president of the Republican State Leadership Committee.

Republicans spent more than $30 million through REDMAP to help elect legislative majorities in states such as Ohio, Michigan, North Carolina, Pennsylvania and Wisconsin, Jankowski said.

“We’re not talking about 2-month-long broadcast buys on network TV that never stop, like you see in a U.S. Senate battle,” Jankowski said. “We’re talking about cable, radio, mail, ground game — very basic stuff.”

Before the 2010 election, the GOP had majorities in 36 state legislative bodies. Afterward, the party controlled 56, according to the National Conference of State Legislatures. In almost half the states, Republicans won control of the entire redistricting process.

To help analyze voting patterns in congressional districts, The Associated Press divided the votes from the 2012 presidential election into all 435 House districts.

Since Obama got the most votes, you might think he won the most congressional districts. But he didn’t.

Nationally, Obama received nearly 5 million more votes than Republican Mitt Romney. But in some states, large numbers of Obama’s votes were packed into heavily Democratic congressional districts. As a result, Romney won in 17 more House districts than Obama.

Independent experts give Democrats little chance to retake the House this year. Even beyond Republicans’ redistricting advantage, the party of the president usually loses seats in Congress during midterm elections.

Rep. Steve Israel of New York, who is in charge of the House Democrats’ campaign operation, rejects arguments that Democrats can’t do it, regardless of the map. Jankowski, on the other hand, expects Republican candidates to continue enjoying the fruits of redistricting.

Still, Jankowski notes that people move and populations change. As the decade wears on, the political benefits will diminish and another redistricting battle will loom.

“It has a shelf life to it and it’s usually not the full 10 years,” Jankowski said. “That’s the reason we have a census every 10 years.”