Christie Picks Jeffrey Chiesa to Be Interim Senator Gov. Chris Christie of New Jersey on Thursday announced the appointment of Jeffrey S. Chiesa, the state's attorney general, to fill the Senate seat left vacant by the death of Frank R. Lautenberg. The appointment comes as Democrats and Republicans across the state scramble to line up support, raise money and secure enough signatures ahead of Monday's deadline to compete in the August primary. Mr. Christie, who earlier this week announced that a special election would be held in October to fill the seat, made his announcement at an afternoon news conference in Trenton. Mr. Chiesa served as chief counsel to Mr. Christie from January 2010 through December 2011, when he was nominated to be New Jersey attorney general. He will not seek the Senate seat when his term is done, Mr. Christie said. The decision promised to shift the dynamics of the race, especially for Republicans. On the Democratic side, Representative Rush Holt, who has represented central New Jersey for eight terms in Congress, on Thursday announced his intention to compete for the seat. He is likely to face Mayor Cory Booker of Newark, a prodigious fund-raiser who has become one of the state's most visible politicians. He had been planning to run for the Senate even before Mr. Lautenberg died. Representative Frank Pallone Jr., a Democrat who has served 13 terms, is also expected to run. He has millions of dollars in campaign funds available for the race. The Republican primary picture remains more uncertain. Democrats in the state outnumber Republicans by 700,000 among registered voters. Steve Lonegan, a conservative who competed with Mr. Christie in the 2009 primary for governor, has announced he would run for the seat. Other names that have been mentioned as possible appointees or candidates include Lt. Gov. Kim Guadagno; former State Senator Bill Baroni, who currently serves as the deputy executive director of the Port Authority of New York and New Jersey; and David Samson, who is the chairman of the Port Authority. Mr. Lautenberg died on Monday, and the next day, Mr. Christie announced plans to hold a primary election on Aug. 13 followed by a general election on Oct. 16. His decision to hold the election before his own Nov. 5 contest, at an estimated additional cost of $24 million, drew fierce criticism from Democrats, as well as some Republicans, who said it was an unnecessary expenditure. The practical impact of the decision has been to force potential candidates to decide quickly whether they want to compete for the seat and then adjust their tactics to navigate a compressed campaign schedule. Republicans and Democrats hoping to run must file petitions by Monday afternoon and must include 1,000 signatures from supporters. Mr. Holt announced his intentions in an e-mail to supporters Thursday morning. "Today, I ask for your support as I seek to serve as your Senator in that seat," Mr. Holt wrote. The reason is simple: I believe I am the best candidate to continue the passionate advocacy for progressive values that Sen. Lautenberg exemplified. Correction: June 6, 2013 An earlier version of this article misspelled the surname of the chairman of the Port Authority of New York and New Jersey. He is David Samson, not Sampson.