Republican John McCain vowed to unite Republicans now that he is the party's likely presidential nominee and said he would uphold conservative principles on national security, taxes and the judiciary.

"I am proud to be a conservative," the Arizona senator said in a speech today at the annual Conservative Political Action Conference in Washington. "You need not examine only my past votes and speeches to assure yourselves that they are my genuine convictions. You can take added confidence from the positions I have defended during this campaign."

McCain's path to the Republican presidential nomination was largely cleared today when his chief rival, former Massachusetts Governor Mitt Romney, announced at the same event that he was ending his campaign.

McCain, 71, was facing a skeptical audience, some of whom booed when his speech was mentioned earlier in the day, and was jeered by a few members of the audience when he talked about his stance on immigration.

He said he is "acutely aware" that he won't be able to bring the party's factions together or prevail in the November general election "without the support of dedicated conservatives."

The coming general election campaign against the Democratic nominee, whether it's New York Senator Hillary Clinton or Illinois Senator Barack Obama, won't turn on minor differences, he said.

"We are arguing about hugely consequential things," McCain said.

Among those, McCain said, are whether government will expand under the Democrats or shrink under a Republican president and whether taxes will remain low.

He also said national security, particularly the war in Iraq and the confrontation with Iran, demonstrate crucial differences.

"Senator Clinton and Senator Obama will withdraw our forces from Iraq based on an arbitrary timetable designed for the sake of political expediency," McCain said. "I intend to win the war."

The Democratic candidates, he said, "won't recognize and seriously address" the threat from Iran's pursuit of nuclear weapons.

McCain said he "won't pretend" that he hasn't broken with other conservatives on some issues, citing his support for an immigration law overhaul that would have established a path toward citizenship for some undocumented workers.

He said he accepted the widespread opposition to his position and promised that among his "highest priorities" would be securing U.S. borders.

During the campaign, Romney had sought to portray himself as the true conservative Republican in the race and suggested that McCain's positions tracked more closely with Democrats. McCain's conservative credentials also have come under fire in recent days from radio talk-show host Rush Limbaugh. James Dobson, chairman of Colorado Springs, Colorado-based Focus on the Family, a family values-focused group, announced he wouldn't vote for McCain.