California’s congressional Democrats ran a secret effort this year to manipulate the work of the independent citizens panel that drew the state’s new political districts, foiling the intent of reformers who sought to remove the redistricting process from the control of party bosses.

Democrats met behind closed doors at the party’s Washington, D.C., headquarters, hired consultants, drew their ideal districts and presented maps to the panel through proxies who never disclosed their party ties or “public interest” groups created specifically for the purpose. In many cases, the panel responded by doing just what the Democrats wanted.

The New York-based nonprofit investigative foundation ProPublica released findings Wednesday from a months-long reconstruction of the Democrats’ stealth redistricting strategy, relying on internal memos, emails, interviews and map analyses.

The success of the strategy has Democrats projecting they might pick up as many as seven congressional seats in 2012 under new district boundaries adopted in the summer, far more than had been expected.

“Every member of the Northern California Democratic Caucus has a ticket back to D.C.,” crowed one internal memo. “This is a huge accomplishment that should be celebrated by advocates throughout the region.”

Democrats this week sought to downplay their role, with Rep. Zoe Lofgren, D-San Jose, saying the final maps were “entirely out of the hands of the members” of Congress.

But California Republican Party Chairman Tom Del Beccaro cited ProPublica’s investigation Wednesday as he called for an immediate investigation of improper partisan influence.

“The redistricting process was hijacked,” he said. “No fair-minded person can now say the process or the result was fair.”

Redistricting occurs once a decade, after the U.S. census, when states redraw their political boundaries to reflect population shifts. California’s Democratic-controlled Legislature has long dominated the process here, although both parties have used redistricting as much to protect incumbents as to gain political advantage.

But voters, angry at political gridlock, stripped legislators of the power to draw their own lines and congressional districts. In 2008 and 2010, they handed the job to a 14-member citizen panel.

The new rules barred the use of incumbents’ home addresses as a basis of the new districts. The commission later also chose to exclude party registration data from its deliberations, a decision that might have contributed to its inability to spot partisan manipulation.

Congressional representatives were free to submit their own maps and testify openly before the commission. But a House aide, who invited members of Congress to a secret March meeting in Sacramento, warned them against disclosing their interests publicly.

“Never say anything AT ALL about redistricting — no speculation, no predictions, NOTHING,” the email read. “Anything can come back to haunt you.”

Members of the California Citizens Redistricting Commission declined to talk with ProPublica about their decisions on specific districts, citing unresolved lawsuits.

But several said they knew there were attempts to deceive them.

“When you’ve got so many people reporting to you or making comments to you, some of them are going to be political shills,” said Esparto rancher and Commissioner Stanley Forbes. “We just had to do the best we could in determining what was for real and what wasn’t.”

The cloaked Democratic strategy was particularly successful in Northern California, ProPublica found.

According to an internal memo, Democrats believed they could save every incumbent if no district crossed the Golden Gate Bridge and if a new San Joaquin County district pulled in liberal voters from eastern Contra Costa County to ensure its Democratic majority.

The party envisioned the new San Joaquin-Contra Costa seat would go to Rep. Jerry McNerney, a Pleasanton Democrat who was at risk of being drawn out of his district. His former gerrymandered seat straddled the Altamont Pass and no one expected it would survive redistricting.

To counter a GOP push for a more conservative district carved from Central Valley counties, McNerney hired Sacramento mapping consultant Paul Mitchell.

ProPublica linked Mitchell to a Facebook page for a group called OneSanJoaquin, where residents could download maps and sample testimony. Formed in April as the mapping got under way, OneSanJoaquin described itself as a nonprofit, but it isn’t registered as such in any state.

Transcripts show that at least a dozen people submitted to the commission testimony originating with OneSanJoaquin, although it is unclear whether the origins were known to the commission or how much they influenced the final map.

In any case, the commission adopted maps similar to those OneSanJoaquin sought. And within days, McNerney announced he would move to San Joaquin County and seek re-election in a district where he will enjoy a far more favorable party registration lead than his former seat.

If the 2010 election were rerun in his new district, he would have won by 7 points, according to the Democrats’ internal analysis obtained by ProPublica.

“McNerney ends up with safer district than before,” Mitchell’s firm tweeted, after McNerney made the announcement. “Wow! How did he do that?”

McNerney could not be reached for comment.

Mitchell, who represented numerous clients in the redistricting hearings, told ProPublica that voters benefitted from the work done by him and others deeply involved in the redistricting.

The commissioners “knew some of the testimony was being fabricated by outside groups,” he said. “But what were they to do? They couldn’t create a screen of all testimony and ferret out all the biases.”

“The work we did created better maps — regardless of if they came with the additional benefit of helping some local city, union or incumbent that was the client,” Mitchell said.

“My only regret is that we didn’t do more.”