On Friday, George Bush, the US president, said in a speech that prosperity and peace were "in the balance" and urged the Republican party to fight on for the White House.
"I'm absolutely confident that with your help, we will elect a person who shares our principles"
George Bush, US president
"We have had good debates and soon we will have a nominee who will carry a conservative banner into this election and beyond," Bush told attendees at the Conservative Political Action Conference in Washington DC.
"I'm absolutely confident that with your help, we will elect a person who shares our principles."
While Bush did not explicitly endorse McCain or the other candidates Huckabee and Ron Paul, the Texas congressman, he has said he will back whoever wins the nomination.
"Let us go forward, fight for victory, and keep the White House in 2008," he said.
McCain's success on Super Tuesday and the departure of Romney from the Republican presidential race left many of the party's more traditionalist base concerned.
Some fear that he will not adhere to the party's conservative policies on social issues such as gay marriage and abortion and the so-called "war on terror".
However, on Friday McCain won the endorsement of his former rival for the nomination, Fred Thompson, the former Tennessee senator popular amongst the party's traditionalist wing.
Neck and neck
Meanwhile on the Democratic side both Obama and Clinton spent the day campaigning in Washington state, where more than 70 delegates are up for grabs.
Polls have the pair neck and neck in the delegate race - with NBC News channel reporting that Obama was slightly ahead in the count with 861 to Clinton's 855 candidates out of a total of 2,025 needed to win the race, although other polls put Clinton ahead.
Clinton's campaign also said it had gathered more than $8m in campaign donations, after the New York senator acknowledged earlier in the week she had lent her campaign $5m of her own money.
Obama's camp, meanwhile, said it had gathered $7m in donations since Super Tuesday.