WASHINGTON (CNN) -- Two potential 2008 presidential rivals teamed up Wednesday to raise money for four Senate Democratic candidates locked in hotly contested races.

Sen. Barack Obama asked Sen. John Kerry's political supporters to donate to New Jersey Sen. Bob Menendez as well as Senate challengers Claire McCaskill of Missouri, Jim Webb of Virginia, and Rep. Harold Ford, Jr. of Tennessee, in an e-mail solicitation.

Obama, who thanked Kerry for giving him the opportunity to speak at the 2004 Democratic convention, said the two potential presidential hopefuls are joined in their effort to campaign for Democrats.

"In the past two years, he and I have been working tirelessly to win back Congress," Obama said.

Kerry praised Obama's communicating skills on ABC's "This Week" Sunday but brushed aside questions about the potential '08 match up.

"Whatever he wants to do; I mean, look, this is a free country," Kerry said. "I think he's a very interesting and very powerful communicator."

For CNN's coverage of key races, check out America Votes 2006