BOSTON — Mitt Romney raised $6.5 million Monday, giving the former Massachusetts governor — and his opponents — a clear indication there is strong support for him should he decide to run for the Republican presidential nomination, he said.

In a campaign fund-raising experiment, Romney's "National Call Day" at the Boston Convention Center had more than 400 supporters — including more than a dozen Utahns — work the phones, calling their own personal networks of likely donors to start funding Romney's presidential campaign.

"I'm overwhelmed by the support, I'm heartened by the friendships, and I'm optimistic about the future," Romney said, adding that people were still making calls after that $6.5 million total was calculated.

Romney said he was "a day closer" to making a formal candidacy announcement but did not give any clues as to when it would be.

"What happened here today was a big statement about the support my friends and colleagues will give to a potential campaign," Romney said. "Had we raised $80,000 today, that would have made me rethink this. ... What happened today underscores the validity of this effort."

Romney formed a presidential exploratory committee with the Federal Election Commission late last week, which allows him to begin accepting money for a potential White House race. He said the amount raised was not meant to send a message to rival candidates but instead send a message to his family and friends that the momentum needed to run a presidential campaign exists.

"In political fund-raising, money talks but early money screams," said Spencer Zwick, Romney's national finance director and adviser. "We do know that it takes at least tens of millions of dollars to be viable."

Traditionally, supporters would get an invitation to a fund-raiser and be asked to write a check. But Romney's committee took it one step further, asking those who already support him to come with their own Rolodex or holiday card list and ask family members, friends or business associates to support Romney, Zwick said. Callers could take credit card donations right over the phone, or pledges for checks to be mailed to the campaign headquarters.

Callers had a bit of a script to follow and had access to information on Romney's stance on certain issues, but mainly spoke on their own to people they were calling.

"They become an extension of the campaign," Zwick said.