None of the candidates have any real chance of challenging Obama in the Presidential elections. Obama is likely to win unless there are some last minute developments that turn the table in the electoral battle. For example if a terrorist attack similar to the attack on World Trade Centre is launched against USA ten days before the election date, it can swing the vote in favour of the GOP.
Among the existing candidates, the Texas Governor is aggressive and unsettles the rhythm of the Democrats and Obama. He can be nominated by the GOP. But his victory may not be good for USA.