By David Jones and Edith Honan
June 7 (Reuters) - Newark Mayor Cory Booker was expected to announce on Saturday he will run for U.S. Senate in New Jersey in a Democratic primary set for August to fill the seat of the late Sen. Frank Lautenberg, a source close to Booker said on Friday.
The campaign staffer asked to remain anonymous because he was unauthorized to speak publicly about the announcement.
Booker had previously announced his intention to seek Lautenberg's seat in 2014, after ruling out challenging Gov. Chris Christie, a Republican, in this year's election.
After Lautenberg's death on Monday, Christie called a special election to fill the remainder of Lautenberg's term. Democrats immediately criticized the decision, calling it a calculated attempt by Christie to avoid appearing on the same ballot as Booker - among the state's most popular Democrats.
Christie is leading his Democratic challenger, Barbara Buono, by more than 30 points, and is seen as hoping a blow-out victory would leave him well-positioned to seek the U.S. presidential nomination in 2016, and cement his reputation as that rare politician who can govern as a tough Republican executive while maintaining a strong Democratic following.
The special election, including the August primaries and the general election three weeks ahead of the normally scheduled November election, will cost the state about $24 million.
Christie dismissed the Democratic criticism, saying New Jersey should have a voter-approved representative in the Senate as soon as possible.
On Thursday, he appointed the state's attorney general Jeffrey Chiesa, who like Christie is a Republican, to fill the seat until the general special election in October. Chiesa said he would not compete for the seat in the special election.
Booker, mayor of the state's most populous city, is among New Jersey's best-known Democrats.
More than half of voters in New Jersey hold a "favorable" view of him, making the Newark mayor among the state's most popular Democrats, according to a Rutgers-Eagleton poll from last November. But in a head-to-head match-up with Christie, the Republican governor would have defeated Booker 53 to 34 percent, the poll found.
"He comes into this race with an incalculable advantage in that he is better known than anyone else on the Democratic side," said Ross Baker, professor of political science at Rutgers University in New Brunswick. "The many years in which he's been in the public eye as mayor of Newark, has projected him statewide for sure, and well beyond the state of New Jersey."
A former city councilman in Newark, Booker faced the city's long-time mayor, Sharpe James, in 2002 in one of the most hotly contested political races in the country. Booker lost the race but his campaign was the subject of an Oscar-nominated 2005 documentary called "Street Fight."
Booker was elected four years later, in 2006.
He is credited with bringing in $1 billion in new development into Newark, including the city's first new downtown hotel in 40 years and the relocation of major companies, including Panasonic, which is building a new North American headquarters in downtown.
Booker has also helped bolster his national reputation with his prolific use of Twitter and by campaigning for Democrats nationally.
He has also built an unusual alliance with Christie, especially in education. Both men have backed the development of new charter schools in Newark.
Booker will enter the primary race against U.S. Rep. Rush Holt, a seven-term congressman. Rep. Frank Pallone Jr. of Long Branch is also widely expected to enter the Senate race.
Officials with the Holt campaign said they are not deterred by the name recognition that Booker has, noting that 75 percent of Booker's campaign donations are from out of state and that the ultimate winner in the primary will be determined by local voters.
"In many ways Mayor Booker has some strong name recognition around the country, but this is an election of New Jersey voters," said Jackie Cornell-Bechelli, political director for the Holt campaign and the former state director of President Obama's re-election campaign.
Booker's announcement is expected at an 11 a.m. event at the Newark offices of Audible.com, which the city says is its fastest-growing private employer.