Matthew Hale, an associate professor at Seton Hall University who closely follows New Jersey politics, said that aside from an assumption that Mr. Chiesa would hold a tough line on law and order issues, it was hard to know how he might vote on many matters in Washington.

“There is just not a huge amount of information we have on his positions,” Dr. Hale said.

Still, he said, Mr. Chiesa is a safe choice for the governor. “This is someone that Chris Christie knows really, really well, trusts a great deal, and will not be surprised by,” he said.

Mr. Christie said he would appoint a new attorney general on Monday.

The decision on the interim senator 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. In an e-mail to supporters, Mr. Holt said he was “the best candidate to continue the passionate advocacy for progressive values” shown by Mr. Lautenberg.

Mr. Holt is likely to face Mayor Cory A. 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. Another Democrat, Representative Frank Pallone Jr., who has served 13 terms, is also expected to run. He has millions of dollars in campaign money available for the race.

The Republican primary picture remains more uncertain. Democrats in the state outnumber Republicans by 700,000 among registered voters.

Steven M. Lonegan, who competed with Mr. Christie in the 2009 primary for governor, has announced he will run for the seat. Other names that have been mentioned as possible candidates include Lt. Gov. Kim Guadagno and former State Senator Bill Baroni, who is the deputy executive director of the Port Authority of New York and New Jersey.