Will the Massachusetts Senate race be the costliest race in the country?

US Sen. Scott Brown raised about $5 million in the second quarter of 2012, less than the more than $8.6 million raised by Democratic rival Elizabeth Warren during the same three months.

Brown's campaign released the total Wednesday, saying that about two-thirds of his campaign donors who gave during the quarter are from Massachusetts. The campaign didn't say how much of the $5 million came from inside the state.

Warren has also not said how much of the money she collected from May-June came from outside Massachusetts.

Brown's campaign finance director said he was pleased with the level of donations to the Republican incumbent.

"We are extremely grateful for the strong showing of support for Senator Brown from the people at home who know him best," John Cook said in a statement.

The $5 million brings Brown's campaign fundraising total to about $17 million when added to figures previously posted by the Federal Elections Commission.

That's compared to the more than $24 million Warren has raised so far this election cycle.

Brown still has an edge in cash on hand, however.

The campaign said it had $15.5 million in its account at the end of the quarter. That's compared with $13.5 million for Warren, a Harvard Law School professor.

Warren's campaign said more than 80 percent of its second-quarter donations were $50 or less, and more than half were $25 or less. The campaign said June was its most successful single month to date, with $3.1 million raised during the month.

A full breakdown of the fundraising totals from both candidates was not immediately released by either camp. Warren's campaign said they plan to file her full report with the FEC electronically on Sunday.

An aide to Warren also said tens of thousands of Massachusetts residents have already contributed to the candidate.

"So far, more than 40,500 residents of Massachusetts have donated to Elizabeth's campaign," Warren spokeswoman Alethea Harney said in a statement Wednesday. "Her proven record of standing up for middle class families is building strong grassroots support across the commonwealth."

Brown's campaign said the election won't be decided by which candidate raises the most money, but on "two very different visions for our future."

"Professor Warren may have the support of the out-of-state, left-wing interest groups, but no amount of money can change the fact that she supports job-destroying tax hikes," Cook said.

The enormous sums being raised by Brown and Warren virtually guarantee the race for the Senate in Massachusetts will be one of the costliest — if not the costliest — in the country.

Unlike other states, however, the Massachusetts campaign has been largely free of attack ads by third-party groups — and will remain so if a deal signed by both candidates holds.

Under the agreement, a candidate who benefits from a third-party group's advertisement has to pay half the cost of the ad to a charity named by the other candidate. To date, Brown has paid nearly $36,000 to Warren's charity. Warren hasn't had to write a check.

Warren is challenging Brown in the November election. Brown won a special election in January 2010 to succeed the late Democratic Sen. Edward Kennedy.