Former Massachusetts governor Mitt Romney's decision to abandon his presidential campaign makes it a virtual certainty that Arizona Senator John McCain will be the Republican Party's presidential nominee this year. But as VOA National Correspondent Jim Malone reports, McCain still has some serious work to do to unify the party behind his candidacy.

McCain's challenge played out before an annual conference of conservative activists in Washington.

Conservatives have long clashed with McCain over his initial opposition to President Bush's tax cut program and his support for measures that would establish a path to citizenship for illegal immigrants.

With those past differences in mind, conservative radio talk show host Laura Ingraham raised doubts about McCain in a speech to the Conservative Political Action conference.

"I do not think it is enough to say that you were a foot soldier in the Reagan revolution," said Ingraham. "I think the question is, what have you been doing for conservatism lately?"

Later McCain was greeted by the same group with a mixture of jeers and cheers. He expressed the hope that the Republican Party will unite behind him as the presidential nominee in time for the November election.

"Many of you have disagreed strongly with some positions I have taken in recent years. I understand that," he said. "All I ask of any American conservative, moderate, independent or enlightened Democrat is to judge my record as a whole and accept that I am not in the habit of making promises to my country that I do not intend to keep."

President Bush also addressed the conservative conference. Although Mr. Bush did not endorse McCain, he did urge conservatives to unite behind the eventual Republican nominee.

"This is an important election," he said. "Prosperity and peace are in the balance. Let us go forward, fight for victory and keep the White House in 2008. God bless you, and God bless America."

Even as McCain moves closer to securing the Republican nomination, many experts believe that the voices of discontent on the right are unlikely to quiet down anytime soon.

Political expert Stuart Rothenberg says that makes for a strange dynamic between the apparent nominee in waiting and an important constituency within the Republican Party.

"When you watch this a long time, you think that the [Republican] base really dictates who the nominee is going to be. But here we have this strange situation where Republicans, conservatives, supporters of George W. Bush, are not enthusiastic about John McCain," he said.

Rothenberg was a guest on VOA's Talk to America web chat, as was political science Professor Bruce Miroff of the State University of New York at Albany.

"Conservatives expect their politicians to be in touch with orthodoxy, and McCain is just too much of a heretic for many conservatives on some key issues," said Miroff.

Miroff says many conservatives will never forgive McCain for initially voting against the Bush tax cuts and for his moderate stance on immigration.

McCain also has a history of clashes with religious conservatives, though he has tried resolve those differences in recent years.

McCain argues that his record should appeal to both social and economic conservatives because of his long-standing opposition to abortion and his focus on cutting wasteful government spending.

Analyst Rothenberg says many conservatives see a stark contrast between President Bush and Senator McCain.

"The groups that he leaves out are the strong conservatives, the religious conservatives, really the Bush supporters. So, in a sense, McCain continues to be what he was eight years ago. He was the alternative to George W. Bush," he said.

McCain lost out to Mr. Bush in a bitter presidential primary race in the year 2000. The two men later reconciled and McCain has become one of the president's most loyal supporters in pushing for a military victory in Iraq.

Many conservatives also dislike McCain because he has a reputation as a maverick Republican who has been willing to work with liberal Democrats on issues such as climate change and campaign finance reform.

Expert Bruce Miroff says McCain's challenge in the coming weeks is to mend fences with his conservative critics without alienating his base of support with moderate and independent voters who have admired his maverick tendencies in the past.

"His problem is that to conciliate the conservatives who now view him with mistrust, he has to sound more conservative, and that clashes with the moderate and independent image, which is his strongest calling card with voters beyond the Republican base," said Miroff.

Recent polls suggest a presidential race between Republican McCain and either of the two Democratic contenders would be competitive. Surveys show McCain either tied or slightly ahead of Hillary Clinton, but trailing Barack Obama by a few points.