In a Senate race that could have implications for the 2008 presidential contest, Sen. Hillary Rodham Clinton (search) raised more than $6 million between April and June.

Campaign reports filed by Senate candidates Friday with the Federal Election Commission showed the former first lady, a New York Democrat, had $12.6 million cash on hand at the end of last month, even though she has no clear opponent yet.

Manhattan lawyer Edward Cox (search), a son-in-law of former President Nixon, has begun preparations to challenge Clinton and Republican party officials also are courting Westchester County District Attorney Jeanine Pirro (search) as a possible challenger.

Patti Solis Doyle, executive director of Friends of Hillary, said she expects "very well-funded opposition next year."

Clinton's first campaign for Senate in 2000 smashed fundraising records, with she and her Republican opponent, then-congressman Rick Lazio, together spending about $80 million.

A strong showing in next year's Senate race could give added momentum to Clinton's early frontrunner status among possible Democratic candidates in the 2008 presidential race. But a bruising contest could slow her down.

Another closely watched Senate race is in Pennsylvania, where incumbent RepublicanRick Santorum (search) is facing a challenge from the son of a former governor.

Santorum raised $3.6 million in three months and reported $5.7 million cash on hand as he prepares to fend off Democratic charges that he is too far to the right on social issues.

The challenger, state treasurer Robert P. Casey Jr., reported raising $1.9 million between April and June, ending the period with $1.6 million cash on hand.

Sen. Bill Nelson (search), D-Fla., raised about $2.2 million, ending June with $5 million cash on hand. He has raised more than $6 million in preparation for his 2006 campaign.

Nebraska Democrat Ben Nelson (search) has amassed $2.3 million for his expected re-election bid next year. He took in $867,000 from April through June, according to his FEC report, although he has not formally announced that he's running for a second Senate term.

That dwarfed the nearly $23,000 for Republican challenger Don Stenberg, who didn't enter the Senate race until late April. Republican candidate David Kramer announced his candidacy on June 28 and will not file a campaign finance report with the FEC until the end of September.