By Jonathan Allen, CQ Staff

John McCain , whose political rise owed to his reputation as a political maverick willing to buck his party, will become its official leader tonight when he accepts the Republican presidential nomination.

What has yet to play out is which McCain will show himself most during the long run to Election Day — a McCain who will push and prod his party into rehabilitating a brand that has suffered during the last four years, or a McCain who will need to position himself more in line with the party and make peace with fellow lawmakers that he frequently angered.

As he defines his vision for the country in his acceptance speech tonight, voters may get a sense of which side has given more ground in the marriage.

His basic challenge, according to political experts, is to clarify that vision and contrast it with that of Democratic nominee Barack Obama .

“He not only needs to give them a reason not to vote for Obama, because that’s not going to be hard,” said Roger Porter, a professor at Harvard University’s Kennedy School of Government who worked for Presidents Ford, Reagan and George H.W. Bush. “He needs to give them reasons to get enthusiastic and go out and get their friends and neighbors to vote for him.”

All week, McCain’s surrogates have presented him as a man of character, experience and judgment who knows when to cut deals with adversaries in Washington and abroad and when to buck his own friends as a matter of principle.

“He’s not afraid to tell you when he disagrees. Believe me, I know,” President Bush said in a Tuesday speech delivered to the convention via satellite from Washington. “This man is honest and speaks straight from his heart.”

McCain policy director Doug Holtz-Eakin said “Senator McCain has been successful at getting bipartisan legislation through the U.S. Senate in his career, and he’s done it because he has a very fascinating combination of two things: number one is a willingness to work with people who may not share every ideal principle with him, but where on that issue, there’s some commonality, and he finds a way to build on what’s in common,”

“The second thing is, he’ll say no. And he’s willing to say no,” Holtz-Eakin said.

In addition to defining himself, McCain must rally the Republican base, a tough challenge for a veteran lawmaker who has so often been at odds with various portions of the party orthodoxy.

That job was made easier by McCain’s surprise selection of the conservative Palin as his running mate.

Republicans hope that McCain, who decidedly does not have a reputation as a dynamic orator, will be able to feed off of Palin’s energy when he takes the stage a night after her speech to the convention.

“I believe this is not a competition between the top of the ticket and his running mate,” said J.D. Hayworth, a talk radio host and former Arizona congressman. “It’s a collaboration.”