Preparing for a potentially serious re-election challenge next cycle, Sen. Hillary Rodham Clinton (D-N.Y.) raised $1.8 million from April 1 to June 30 — more than double the take of any of the other 31 Senators up for re-election in 2006.

Since the start of 2001, Clinton has raised nearly $9 million through her Friends of Hillary campaign committee, a total that has been eclipsed only by the roughly $10 million raised by Sen. Maria Cantwell (D-Wash.) as she attempts to retire a massive personal debt.

At last month’s Democratic convention in Boston, Clinton told The Associated Press she “would raise whatever money I need to raise.”

In the 2000 open-seat contest against then-Rep. Rick Lazio (R), Clinton raised and spent $41 million. Lazio matched that spending, making the race the most expensive Senate contest in history that didn’t involve a self-funding candidate.

Howard Wolfson, a Clinton adviser, said that “her race is coming up in the next cycle, and national Republicans are already making clear that they intend to target her.”

Clinton’s fundraising prowess “speaks to the tremendous grassroots support she enjoys,” Wolfson added.

Republicans have said defeating Clinton is one of their top priorities in 2006, but neither of their top two candidates — former New York City Mayor Rudy Giuliani and New York Gov. George Pataki — seems interested in the race.

Giuliani is expected to turn his attention either to the open governor’s seat in 2006 or to laying the groundwork for a presidential bid in 2008. Pataki, too, seems more interested in a presidential race than a run against Clinton.

And, if neither man runs, which seems likely, the Republican bench in the state is extremely thin. After casting about for more than a year, Republicans finally settled on little-known state Assemblyman Howard Mills as their preferred challenger to Sen. Charles Schumer (D).

A look at the July quarterly Federal Election Commission filings of the other 31 Senators who are up for re-election in 2006 should soothe Democrats worried that six of the party’s Senators will be at least 70 years old and will have served at least two terms by Election Day 2006.

At least two septuagenarians — Sens. Dianne Feinstein (Calif.) and Edward Kennedy (Mass.) — appear set to run for re-election, judging by their recent fundraising activity.

Feinstein raked in $723,000 from April 1 to June 30, more than double what she raised in the first three months of 2004.

“She is running in 2006,” said Feinstein Chief of Staff Mark Kadesh. “California is a large and expensive state, and Senator Feinstein has decided to do [the fundraising] not just in the last two years but over a longer period, so it is a bit easier.”

Feinstein was Democrats’ top choice to run in the 2003 gubernatorial recall election, but she demurred, choosing instead to support the unsuccessful attempt by then-Gov. Gray Davis (D) to fight the recall.

She has been mentioned as a challenger to Gov. Arnold Schwarzenegger (R) in 2006, but most observers consider that a remote possibility.

The $567,000 raised by Kennedy in the second quarter also seems to put rumors to rest that he might bow out. Kennedy, who was first elected in 1962, cruised to a seventh term in 2000 with 73 percent.

“This signals his intention to run again,” said Jim Manley, a spokesman for Kennedy. “The fundraising this year is mostly direct mail, which he traditionally does in the off-years — designed in part to build a base for 2006.”

Manley added the vast majority of Kennedy’s time this year has been dedicated to raising money for the presidential campaign of fellow Massachusetts Sen. John Kerry as well as for the Democratic National Committee and the Democratic Senatorial Campaign Committee.

Republicans are considered unlikely to field a top-tier candidate if Kennedy runs. Much of the attention in the state will be directed instead at a 2005 special election to fill Kerry’s seat in the event he is elected president this fall.

But the signs were more mixed among the fundraising filings of other aging Democratic Senators.

Leading that list is Maryland Sen. Paul Sarbanes (D), who will be 73 in November 2006 and has done nothing to shake off persistent retirement rumors. He raised just $22,000 from the beginning of April to the end of June, with $51,000 on hand.

While Sarbanes’ fundraising was relatively meager, it dwarfed the quarterly totals of Hawaii Sen. Daniel Akaka ($50 raised) and West Virginia Sen. Robert Byrd ($700 raised).

Both Byrd, who will be 88 in November 2006, and Akaka, who will be 82, have signaled they will run for re-election, however, and both appear untouchable in their home states.

Wisconsin Sen. Herb Kohl (D) raised just $82,000 in the period, $80,000 of which came from his own pocket.

The most famous self-funder in the Senate, New Jersey Sen. Jon Corzine (D), also has shown little hesitancy about dipping into his own funds to provide seed money for a 2006 campaign.

In his July quarterly report, Corzine showed a $250,000 personal donation, bringing his cycle-to-date investment to $1.95 million.

Subsequent FEC filings reveal that Corzine chipped in another $150,000 to the campaign on July 23.

In his open-seat race four years ago, Corzine spent more than $60 million of his own money, winning the contest with 50 percent.

He is now being mentioned as a leading candidate for the 2005 gubernatorial race, following the surprise resignation of Gov. James McGreevey (D) on Thursday.

Among potential Republican retirees, Senate Majority Leader Bill Frist (Tenn.) continued to give no signs that he would back away from leaving the Senate in 2006 in accordance with a self-imposed two-term limit.

Frist raised no money for his campaign account from April 1 to June 30 — the only Senator with that unique distinction. He ended the period with $383,000 on hand.

Frist continued to be especially active, however, through his leadership political action committee, raising $3.6 million in the first 18 months of the 2004 cycle.

The man Frist replaced as leader of the Senate Republicans, Mississippi Sen. Trent Lott, raised a paltry $19,000 through his campaign committee but has brought in nearly $3 million for his leadership PAC since being ousted as leader at the end of the 107th Congress.

Lott had been viewed as a likely retiree following his demotion, but said he would “keep his options open” for 2006.

Texas Sen. Kay Bailey Hutchison (R), rumored to be a potential gubernatorial candidate in two years, continued to hold the largest war chest of anyone in the class of 2006.

She ended June with $6.6 million in the bank — a sum she could transfer in its entirety to a state account for a governor’s race.

Second only to Hutchison’s war chest was that of Clinton, who closed June with $4.1 million on hand.

Clinton opened her “Friends of Hillary” Web site for the 2006 campaign last summer. At that time, she had raised $2 million for her campaign committee, a total she has tripled in the year since.