Sen. Barbara Boxer is aggressively raising money in anticipation of a tough 2010 reelection bid that could pit her against the political star-power of Arnold Schwarzenegger.

The California Democratic senator reported $3.5 million in cash on hand, compared with just $1 million at a similar point before she won her third term in 2004, according to campaign filings.

“I think it’s also a good thing to send a message to whoever runs that it’s going to be a very hard race for them,” said Boxer, well-known as an anti-war, staunchly pro-abortion rights and outspoken environmentalist, in an interview last week. She noted that she made a “mistake” in past election cycles by waiting too long to raise money.

Her goal this time around is to raise at least $20 million, and she’s well ahead of past campaigns.

In the two-year period before she won her second term in 1998, she racked up $2.1 million, and in the years before she was first elected to the Senate in 1992, she secured a meager $37,000, according to records compiled by the database CQ MoneyLine.

Even if the action-hero-turned-governor stays out of the race, Boxer could face other candidates who could bankroll campaigns from their personal fortunes. According to GOP and Democratic operatives in the state, the Republican short list includes two senior advisers to John McCain’s presidential campaign — Carly Fiorina, the former chief executive officer of Hewlett-Packard, and former eBay CEO Meg Whitman — as well as Steve Poizner, the state’s insurance commissioner who in 2000 sold his technology company to Qualcomm for $1 billion.

Boxer could be spared if Fiorina, Whitman or Poizner choose to run for governor in 2010, or if Schwarzenegger receives a position in a new administration. In the meantime, she is trumpeting her fundraising numbers to send a warning that she will be tough to knock off in the next election cycle.

“My concern doesn’t land on any particular individual,” she said. “But I certainly have to be prepared in case Arnold runs, and I certainly have to be prepared in cases where others run who are independently wealthy.”

The senator isn’t relying on money alone to gain an early edge. She’s tapping into star-power on the left, holding fundraisers with singer Bonnie Raitt, folk rocker Jackson Browne and comedian Larry David, the co-creator of “Seinfeld.”

Her campaign is bankrolled by nearly 22,000 individual donors, 82 percent of whom contributed $100 or less. The Hollywood elite has also cut her big checks, including some who have maxed out on the $2,300 contribution limit for the primary and $2,300 for the general election.

The list of donors reads like a stroll along Hollywood Boulevard: $4,600 from Matt Groening, the creator of “The Simpsons”; $4,600 from Richard Donner, director of “Superman” and “Lethal Weapon” movies; $4,600 from Alan Horn, CEO of Warner Bros.; $1,500 from Robert Iger, CEO of The Walt Disney Co.; and $2,000 from actress Barbra Streisand, according to Federal Election Commission records.