A Harvard Law School professor who advised the McCain campaign and once served as a high-ranking official in the Reagan Justice Department announced Friday that he had voted for Democratic presidential candidate Barack Obama, part of a growing trend of leading Republicans crossing party lines to endorse the Illinois senator.

Charles Fried, a leading conservative thinker, cited McCain’s choice of running mate as the reason he cast his absentee ballot for Obama, who graduated from the Law School in 1991. Fried will also resign from his post on the McCain Campaign’s Justice Advisory Committee.

“Professor Fried is concerned about the choice of Sarah Palin when the nation is in crisis,” law professor and Obama adviser Cass R. Sunstein ’75 said in an interview on Friday. “I think his view is that at this time, the prospect of Governor Palin becoming President Palin is a source of concern.”

But at least one conservative at Harvard, Ruth R. Wisse, a professor of Yiddish and comparative literature, said that voters should not cast their ballots based on who the vice presidential nominee is.

“[Palin] might not have been my first choice, but...in U.S. politics, we vote the top of the ticket,” Wisse said in an e-mailed statement. “A vote for Obama seems a foolhardy leap of faith rather than a choice of lesser evils.”

Fried declined to comment on the reasons behind his decision to vote for Obama, beyond referring to a blog post by Sunstein on The New Republic’s Web site that announced the news of Fried’s endorsement.

Sunstein compared Fried’s defection from the McCain campaign to that of former Secretary of State Colin Powell, who endorsed Obama earlier this month.

“I think this is a really big deal in the campaign, and even in American history,” said Sunstein, who was Obama’s colleague at the University of Chicago.

Randall L. Kennedy, another Law School professor advising Obama, said he was “not surprised” to learn that Fried had chosen to support the Democratic candidate.

Both Sunstein and Kennedy noted that Fried was a “thoughtful” member of the right whose views have always been more libertarian than conservative. Fried has, for example, supported keeping abortion services legal.

Kennedy pointed not only to the much-debated choice of Palin as the vice-presidential candidate, but also to the “allegations” and “pandering” of the McCain-Palin campaign as a possible reason for conservatives’ disillusionment with the Republican ticket.

“Given the character of the campaign McCain and Palin have waged, it does not surprise me that a person like Charles Fried would be alienated,” Kennedy said. “I’m glad that my colleague has seen fit to support the better candidate.”

In recent weeks, Obama has collected endorsements from a slew of moderate Republicans, including former Bush press secretary Scott McClellan, as well as the former Republican governors of Massachusetts and Minnesota, William F. Weld ’66 and Arne H. Carlson.

­—Staff writer Athena Y. Jiang can be reached at ajiang@fas.harvard.edu.