(CNN) -- Sen. John McCain Thursday told a conservative-rich audience that he has what it takes to unite the Republican party.
"I know I have a responsibility, if I am, as I hope to be, the Republican nominee for president, to unite the party and prepare for the great contest in November," McCain told the Conservative Political Action Conference.
Critics say he's too liberal to carry his party's nomination. Conservatives point to him breaking with the party on immigration, opposing the Bush tax cuts and co-sponsoring legislation on campaign finance reform.
The meeting is the nation's largest annual gathering of conservative activists, students and policymakers, according to CPAC.
The speech came just hours after McCain's chief rival, former Massachusetts Gov. Mitt Romney, announced he was suspending his campaign. Watch McCain talk about his "conservative convictions" »
McCain, locked in as the front-runner for the GOP presidential bid, has yet to secure the support of his party's conservative side. Watch how conservative McCain's record is »
The Arizona senator said he was proud of his conservative record and said he was thankful for the opportunity "to make my case." Watch what McCain says about his relationship with conservatives »
"My record in public office taken as a whole is the record of a mainstream conservative," McCain said, calling attention to positions he said he has defended during his campaign.
McCain pointed to campaigning in Iowa in opposition to agriculture subsidies, campaigning in Iowa against government-mandated health care and campaigning in Michigan for tax incentives.
On the issue of immigration, McCain received a mixed response from the audience, with boos turning to cheers.
"Surely, I have held other positions that have not met with widespread agreement from conservatives. I won't pretend otherwise nor would you permit me to forget it," he said.
Unlike his original immigration proposal that included a path to citizenship, McCain said he would secure the borders first before offering other ways to deal with illegal immigration.
"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," he said.
The crowd responded enthusiastically when McCain said he would make the Bush tax cuts permanent. McCain voted against the president's first two tax cuts in the Senate.
Further trying to separate himself from the liberal ideas he has been associated with, McCain detailed what he called "significant differences" between him and the Democratic candidates, Sens. Hillary Clinton and Barack Obama.
McCain also asked the audience to forgive him for his absence from last year's CPAC, when he was the only major Republican candidate to reject an invitation to speak at the conference.
"I hope you will pardon my absence last year, and understand that I intended no personal insult to any of you," he said. "I was merely preoccupied with the business of trying to escape the distinction of preseason front-runner for the Republican nomination, which, I'm sure some of you observed, I managed to do in fairly short order."
Ahead of his speech, McCain sent a letter to the Federalist Society, an influential conservative group. In it he promised to nominate "judges who understand that their role is to faithfully apply the law as written, not impose their opinions through judicial fiat."
"When I was running for president in 1999, I promised that, in appointing judges, I would not only insist on persons who were faithful to the Constitution, but persons who had a record that demonstrated that fidelity," the letter said.
In an e-mail to CNN, a conservative activist said the letter had generated serious "buzz" in conservative legal circles, where the letter is seen as a strong commitment. One prominent conservative lawyer, speaking on condition of anonymity, called it a "strong statement."
McCain is a conservative, just not always a politically correct conservative, said CNN senior political analyst Bill Schneider.
"He's a conservative, he's been a conservative for a long time, but on occasion he departs from the conservative orthodoxy," Schneider said.
"He is now advertising his ability to make bipartisan deals on issues because that's what voters seem to want this year," he said.
But McCain did win key GOP states like California and Missouri this week. In the all-important Super Tuesday contests, McCain took at least 504 delegates, compared to 175 for Romney and 141 for Huckabee.
Less than half -- 49 percent -- of Tuesday's voters who said they voted for McCain described themselves as conservative, exit polls from the 15 states holding GOP contests showed.
About 80 percent of Romney voters described themselves as conservative, compared with 75 percent for Huckabee, the initial exit polls showed.
McCain said he's confident he can bring the party together and said he's not worried that some "very conservative" Republicans don't think he is conservative enough.
"Is there a lot of work to unite the entire party?" he asked rhetorically. "Sure. After the campaigns are over, you've always got the task of uniting the party behind the nominee."
But he expressed confidence he can do just that. "Our message will be we all share common conservative principles," he said. "Fundamental conservative political philosophy, which has been my record."
The battle for the Republican nomination now shifts to Louisiana and Washington state, where voters will cast ballots Saturday. GOP voters cast ballots next Tuesday in Maryland, Virginia and Washington, D.C. E-mail to a friend
CNN's Dana Bash, Carol Costello and Bill Schneider contributed to this report.