BOSTON — Republican Mitt Romney, who has a net worth estimated at up to $350 million, said Monday he has given money to his presidential campaign for a second time and may do so again before this weekend's fund-raising deadline.

The former venture capitalist declined to say how much money he has contributed, although the financial reports he has to release within weeks will reveal the amount.

The second-quarter fund-raising period for all candidates ends Saturday.

Romney said an unprecedented $4 million that he spent on early advertising forced him to make the personal contributions. The former Massachusetts governor served only one term and started the campaign largely unknown outside his home state.

"One of the things I've had to do that you don't have to do if your name is McCain or Giuliani is introduce myself to people in Iowa and South Carolina and New Hampshire," Romney said, referring to rivals John McCain and Rudy Giuliani.

Romney revealed the donation as his supporters sought contributions from their friends and business colleagues at an event in TD Banknorth Garden, home to the Boston Celtics and Bruins.

Romney sought to lower expectations, telling reporters, "I don't know how much we're going to raise here. If we do a million, that would be terrific. If we do more than that, that would be fabulous."

He also predicted his second-quarter report would show he had raised substantially less during the quarter than his total for the first three months of the year. His staff cited time spent on campaign appearances and debates. Interest among conservative donors in Fred Thompson, who is expected to announce his own campaign next month, was also a factor.

During his first "National Call Day" in January, Romney raised $6.5 million, helping to propel him toward a record $21 million in fund raising. He also contributed $2.35 million to his campaign, giving him a first-quarter total of $23 million.

At the time, Romney said "it would be akin to a nightmare" if he had to donate to his committee — even though he had already done so when he made that comment to reporters. He later said the donation was necessary as seed money.

On Monday, he would not say if he had made loans or outright donations. He is not limited in how much he can give to his own campaign.

In his first comments, Romney also said he supports a top aide, Jay Garrity, who is under investigation in Massachusetts and New Hampshire for impersonating a law enforcement officer. Garrity took a leave of absence from the campaign on Friday.

"I'm going to give him the benefit of the doubt on this, and I think other people would be wise to do the same thing," Romney said. "He's a good guy and wish him the very best, but this is really now in his hands."