Colin Powell is the most high-profile Republican to endorse Senator Obama
Republican solidarity appears to be at stake as more prominent Republicans have lent their support to Democrat Senator Barack Obama in the campaign.
Former Senator Charles Mathias of Maryland is the latest to throw his weight behind the Democratic nominee.
He joins former Secretary of State Colin Powell, former Massachusetts Governor William Weld, and former Bush press secretary Scott McClellan.
The selection of Sarah Palin is one factor turning off some Republicans.
Former Senator Mathias told The Washington Post:
"For me, the decision is based on the long-range needs of our country and which of these two candidates I feel is better suited to recharge America's economic health, restore its prestige abroad and inspire anew all people who cherish freedom and equality.
"For me, that person is Barack Obama."
The economy is a major consideration for voters in this election. A Gallup poll found that not only is the economy perceived as the most important issue in the election, but the percentage who rate it as extremely important to their vote is the highest since 1996.
It is too early to determine the impact of the Obama endorsements
The poll found that although the economy was the top-rated issue for all voters, Republicans gave more importance to terrorism and moral values than did Democrats.
These results came after former secretary of state Colin Powell's public endorsement of Mr Obama last week.
'Alienated' the middle
Mr McCain said he was not surprised by Mr Powell's move, and highlighted the secretaries of state who do back him.
"I'm also very pleased to have the endorsement of four former secretaries of state, Secretaries [Henry] Kissinger, [James] Baker, [Lawrence] Eagleburger and [Alexander] Haig. And I'm proud to have the endorsement of well over 200 retired Army generals and admirals", Mr McCain said.
Most recently, former Massachusetts Governor William Weld, who supported Mr Romney during the primaries, announced his endorsement of the Democratic candidate.
President Bush's former speechwriter David Frum indicated that Mr McCain's selection of Sarah Palin as his running mate, could be a reason for Republican disunity.
"The very same campaign strategy that has belatedly mobilized the Republican core has alienated and offended the great national middle, which was the only place where the 2008 election could have been won", Mr Frum said.
Republican Congressman Christopher Shays, who is the co-chairman of the McCain campaign in Connecticut, told the Yale Daily News that Mr McCain "did not live up to his pledge to fight a clean campaign."
And Minnesota Governor Tim Pawlenty, once considered a possible vice-presidential choice for John McCain, said that Obama "has a pretty good advantage in Minnesota right now."
Republican leaders are not alone in crossing over party lines to support the Democrat.
Some traditionally conservative newspapers have turned their backs on Mr McCain. The Chicago Tribune and The Denver Post, which endorsed President George W. Bush in 2004, threw their support at Mr Obama.
The Los Angeles Times, who backed Mr McCain during the Republican primaries, also endorsed Mr Obama for president.
But how much influence such endorsements have on the final outcome is not clear.
The latest opinion polls show the race tightening. A Rasmussen poll has Mr McCain narrowing the gap to within three points of Mr Obama, at 47% to 50%.