More than 20 leading social conservatives signed an open letter to presumptive Republican nominee John McCain expressing their opposition to the idea of having Mitt Romney as vice president on the Republican ballot.

The so-called "M&M" ticket is being pushed by top Republican strategist Karl Rove, conservative news host Sean Hannity and others in the economic wing of the Republican Party.

But opponents warned that having Romney - the former Massachusetts governor who was pro-choice and pro-gay rights - on the party's ticket could keep value voters at home this fall.

"If Governor Romney is on your ticket, many social conservative voters will consider their values repudiated by the Republican Party and will either stay away from the polls this November or only vote down ticket," warned signers of the letter which will appear in full page ads running in cities which McCain is campaigning in, beginning last Saturday in Prescott, Arizona.

"For the sake of your election, the health of your party, and the future of America you must not allow the obvious electoral consequences of that to occur," concluded the text of the ad addressed to McCain.

Among the signatories is Paul Weyrich, a prominent social conservative and head of the Free Congress Foundation, who had previously supported Romney but now is said to "publicly regret it".

Other signatories include Matt Barber of Concerned Women for America, Michael W Calsetta of Conservative Democratic Alliance, Janet Folger of Faith2Action, Peter LaBarbera of Republicans for Family Values, and Stephen Strang of Strang Communications.

The letter comes at the heels of McCain's announcement this past week that he has turned over a list of about 20 names to a group that will lead his vice presidential search team.

On the Don Imus radio programme on Wednesday, 71-year-old McCain quipped that his choice of vice president would be especially crucial because of his age, according to Agence France-Presse. If elected McCain would be the oldest first-term US president.

The Arizona senator noted that he would like to have his vice president set before the Republican convention in St Paul, Minnesota, in early September when his party is expected to officially nominate McCain as the party's choice to face either Democrat Barack Obama or Hillary Clinton.