The campaign of Sen. Hillary Rodham Clinton (D-N.Y.) spent a stunning $4 million in the third quarter of the year, part of what aides call an effort to build an even bigger national donor base for the likely presidential aspirant.

The lion’s share of that amount, $3 million, was spent on efforts to expand the number of small-dollar donors through direct mail and the Internet, according to aides.

Officially, the direct-mail push is an effort to continue to build a big enough campaign war chest to bolster Clinton’s re-election effort in 2006, for which she has already raised more than $27.2 million in less than five years in the Senate, according to filings with the Federal Election Commission.

But, with no top-tier opponent challenging her and $13.8 million sitting in her account as of Sept. 30, Clinton’s free spending on donor prospecting could just as easily be interpreted as an effort to build the sort of national network of donors, large and small, that would be needed to fund a presidential campaign in 2008.

Clinton’s top campaign operative shrugged off any suggestions that the major foray into direct mail was anything out of the ordinary, and noted that it continues to pay dividends.

“We made a serious investment in direct mail and are very pleased with the results,” said Patti Solis Doyle, executive director of Friends of Hillary.

The Clinton campaign had its best quarter yet among small-dollar donors, receiving more than $2 million in donations of less than $200, according to FEC reports.

In addition, the campaign said there were 70,000 individual contributors, the largest number of donors it had in any quarter. Out of those 70,000 donors, 95 percent gave $100 or less.

Most importantly, the campaign said the investment in direct mail — which is notoriously expensive — paid off in the form of 47,000 new donors to Clinton in the third quarter, which ran from July 1 to Sept. 30.

The $4 million Clinton spent from July through September was far and away the most of any Senate campaign. No other Senate candidate for 2006 spent $1 million in the third quarter, with Sen. Rick Santorum (R-Pa.) and his opponent, state Treasurer Bob Casey Jr. (D), the only other candidates to top $800,000 in expenses for the third quarter.

It was easily the most money Clinton has spent in any quarter, with most recent three-month period’s expenditures representing 30 percent of the entire $13.4 million she has spent on the campaign since entering the Senate in 2001.

Clinton’s supporters note that other politicians have spent large amounts on direct mail in Senate races, pointing particularly to the aborted Senate campaign of former New York City Mayor Rudolph Giuliani (R). In the first quarter of 2000, Giuliani spent more than $3.5 million on direct mail on his way to spending more than $5.6 million over those three months.

But Giuliani was taking on a national figure in Clinton, trying to tap into the conservative activist’s anger with her tenure as first lady.

For now, Clinton is facing low-level competition in Westchester County District Attorney Jeanine Pirro (R), whose campaign has been dogged by missteps from the start and ended September with just $364,000 in its account.

A huge small-dollar donor base will serve Clinton well if she does decide to run for president in 2008; those donors proved to be the financial engine for Democrats in the 2004 presidential races. While previous presidential campaigns focused on garnering as many maximum donations as possible, former Vermont Gov. Howard Dean (D) rode a wave of smaller donations, largely gathered through the Internet, to a total haul of more than $50 million. And the eventual nominee, Sen. John Kerry (D-Mass.), also saw a massive influx of small-dollar contributions that essentially leveled the financial playing field between his campaign and President Bush’s, with both campaigns spending more than $250 million.

The former first lady is not content to just raise money for her own campaign. She continues to be the most attention-grabbing figure for Democratic activists, and she’ll demonstrate that cache with a trio of events on Oct. 28 in Boston: First an event raising money for a new joint fundraising committee that is splitting money among several female Senate Democrats; secondly, an event for New Hampshire Gov. John Lynch (D), who’s running for re-election next year; and finally for Gov. John Baldacci (D-Maine).

In addition, she’ll travel to Michigan for events on behalf of Sen. Debbie Stabenow (D) and to Washington for Sen. Maria Cantwell (D), and on to California for the Democratic Senatorial Campaign Committee.