NEWARK, N.J. – Four years after spending more than $60 million on his first bid for public office, Senator Corzine is ready to move on. The former investment banker announced yesterday that he is running for governor in 2005.

“I believe I can do more for the state of New Jersey and its citizens as governor,” Mr. Corzine said during an afternoon news conference at a city hotel.

Mr. Corzine, 57, said he left private life for the Senate to press for better health care, education, and government and pledged to continue those battles.

“I believed five years ago, and I still do, that it’s my responsibility,” Mr. Corzine said.

Mr. Corzine’s announcement made him the first official Democratic entrant. Potential rivals include acting Governor Codey and Rep. Robert Andrews.

Businessman Doug Forrester, who lost a Senate race to Frank Lautenberg in 2002, and Bret Schundler, who lost the 2001 governor’s race to Governor McGreevey, are among the Republicans who have announced their candidacies.

Mr. Codey, who was also in Newark yesterday, said he and Mr. Corzine are friends. “It’s his right to run,” Mr. Codey said. “He considers me a friend of his. So be it. It is what it is.”

Mr. Corzine said he did not consider himself engaged in a rivalry with Mr. Codey, nor did he believe that he was hurting the party by seeking the nomination ahead of an incumbent Democrat.

He said New Jersey had a bipartisan history of corruption, which he vowed to reverse through personal leadership and reforms. For example, he said, “I don’t believe in no-bid contracts.”

But Republicans quickly attacked Mr. Corzine for contributing to elected Democrats who have left office recently either under indictment or a cloud of scandal.

“He funded and embraced the politicians who brought scandals to our state, raised our taxes, and mortgaged our future,” Mr. Forrester said in a statement.

David Rebovich, director of the Rider Institute for New Jersey Politics in Lawrence, said Mr. Corzine must explain his decision to leave the Senate so soon after a 2000 campaign on which the former Goldman Sachs chairman spent much of his personal fortune.

“What Corzine has to do,” Mr. Rebovich said, “is convince not just all New Jerseyans, but Democrats first, why he is interested in being governor.”

Mr. Corzine also has to show that his lack of experience in state politics – he has none – is not an impediment, Mr. Rebovich said.

If Mr. Corzine can’t explain himself adequately, Mr. Rebovich added, “He will be subject to the criticism that he’s a bored billionaire with no power in Washington and he just wants to come back to Jersey.”

If Mr. Corzine wins, he as the new governor would appoint someone to serve out the final year of his Senate term.

Asked whether he would favor Rep. Robert Menendez to take his Senate seat, Mr. Corzine replied that there were seven Democratic House members from New Jersey, and that it was too early to say which one he would choose. One of the seven, Rep. Frank Pallone, issued a statement endorsing Mr. Corzine for governor immediately following his announcement.

Mr. Corzine pledged to complete his four-year term as governor if elected.

But Republican State Chairman Tom Wilson said the wealthy Democrat is driven by ambition, and that his record suggested the governorship was just one more stepping stone to higher office.

“We’ve heard he wants to be president,” Mr. Wilson said.

A spokesman for Mr. Corzine, David Wald, denied the assertion.

“He’s running for governor,” Mr. Wald said. “Period.”