In what may prove a significant development for the 2012 elections, David Brock, a prominent Democratic political operative, says he has amassed $4 million in pledges over the last few weeks and is moving quickly to hire a staff to set up what he hopes will become a permanent liberal counterweight over the airwaves to the Republican-leaning outside groups that spent so heavily on this year’s midterm elections.

Kathleen Kennedy Townsend, a former Maryland lieutenant governor and the eldest of Robert F. Kennedy’s 11 children, has agreed to serve as the chairwoman of the group, which will be called American Bridge, lending to the still extremely nascent undertaking the weight of what remains one of the most significant families in Democratic politics.

Leading Democratic donors who have already pledged money to the group include Rob McKay, heir to the Taco Bell fortune and chairman of the Democracy Alliance, a partnership of wealthy liberal donors; Robert Dyson, who heads Dyson-Kissner-Moran, a takeover and acquisitions firm in New York City; and Marcia L. Carsey, a television producer who gave $1 million to Democratic outside groups in 2004.

Mr. Brock said in an interview that he planned to formally file papers with the Federal Election Commission on Tuesday to set up American Bridge as what is known as an independent-expenditure-only political action committee, meaning it will be able to take in contributions of unlimited size from individuals and corporations but must regularly disclose its donors.