With polls suggesting that Democrats have a chance to end 12 years of Republican rule in Congress, liberal Democrats in Brooklyn are charging up their cell phones and taking advantage of free weekend minutes to call on voters in the most closely contested races around the country.

MoveOn.org, the left-wing political organization that gained national fame by using the Internet to propel the candidacy of Governor Dean for president four years ago, spread its roots to Brooklyn over the weekend with the opening of a campaign office in Carroll Gardens.

The group chose the Brooklyn site as a base to recruit its foot soldiers even though there are no competitive congressional races in the borough, where Democrats have long held sway. MoveOn.org is using a second-floor office as a phone bank for Call for Change, a get-out-the-vote effort consisting of volunteers calling registered Democrats around the country.

The program’s goal is to call 5 million voters in 30 districts nationwide that have been designated as highly competitive. In a training session yesterday at the Carroll Garden’s headquarters, volunteers were versed on the Call For Change philosophy. A representative explained to about a dozen novice callers that tight elections can be decided by four to five percentage points, making voter turnout crucial. If their calls persuade Democrats who might otherwise stay away from the polls that their vote is essential, the election can be swung, volunteers were told.

New York is home to three congressional races on which Call For Change is focusing, all upstate. In order to take control of Congress, Democrats would need a net gain of 15 seats in the House and six seats in the Senate.

Call For Change recruits its volunteers with a strong Web presence and by word of mouth, organizers said. “I became involved after the election debacle in 2000 when a relative got me hip to the organization, and this felt like the logical next step,” a 46-year-old volunteer from Park Slope, Bob Vuolo, said.

With a liberal, largely anti-war message, MoveOn.org has had success already this year by helping a Connecticut businessman, Ned Lamont, defeat an incumbent senator, Joseph Lieberman, in Connecticut’s Democratic primary in August. Mr. Lieberman is now running as an independent, and polls show him leading Mr. Lamont in the general election race.

MoveOn.org can be picky about the candidates it chooses to support. It made no endorsement in Senator Clinton’s primary victory last month, in which an anti-war challenger, Jonathan Tasini, had aggressively sought the group’s backing. Despite Moveon.org’s official neutrality, one volunteer spoke warily of Mrs. Clinton yesterday.”I love Hillary as a mother and as a politician, but she has a lot of enemies and I don’t know if she can win the presidential election.”