Raising images of war and stressing his experience as a Vietnam veteran, Sen. Hillary Rodham Clinton's Republican challenger says in a new fundraising letter that her "ruthless campaign to seize power is a serious threat to a secure future for America."

The letter from former Yonkers Mayor John Spencer asks contributors to return the enclosed "National Security Dispatch" to "Lt. John Spencer (USA-Vietnam)" because "no battle is more important to America's future than stopping Hillary Clinton."

The letter drew a sharp rebuke Wednesday from Clinton adviser Howard Wolfson, who said that "in Mr. Spencer's bizarro world, declaring war on your political opponent is normal."

Wolfson said the letter represented another departure from what Spencer had vowed would be a "positive" campaign. In response, Spencer adviser Rob Ryan said "once again the Clinton campaign throws mud."

Spencer has long maintained that Clinton is using her re-election race as a stepping stone to a presidential run and stresses that theme in his latest appeal for "the ammunition it will take to win" what he calls "this do-or-die battle." Clinton has continually ducked questions about whether she intends to run for president in 2008.

Trailing badly in the polls and in fundraising, Spencer said the Democratic incumbent can be beaten if his campaign is smart about the use of limited resources.