Thirty-three percent (33%) of American voters say they’ll definitely vote for Senator Barack Obama (D) if he’s on the Presidential ballot in November 2008. That’s unchanged from a month ago, but the number who say they’ll definitely vote against the charismatic newcomer from Illinois has increased by five points to 38%. Still, Obama enjoys the highest level of base support and net support among any of the leading candidates.

Obama can take solace in the fact that other Presidential hopefuls have seen their numbers slip a bit in recent months as well. Former New York City Mayor Rudy Giuliani (R) now has 27% who will definitely vote for him and 37% who will definitely vote against. That’s the second straight month that the GOP frontrunner’s numbers have slipped. In March, 31% were planning to definitely vote for him while just 27% were definitely poised to vote against (see trends for all candidates).

In fact, Giuliani’s numbers have slipped to the point where they match the former GOP frontrunner, Arizona Senator John McCain (R). The number who say they’ll definitely vote for McCain has increased in each of the past two months, but so has the number who will definitely vote against.

The Democratic frontrunner’s numbers have barely moved—28% say they’ll definitely vote for New York Senator Hillary Clinton (D) while 47% say they’ll definitely vote against the former First Lady. Only Newt Gingrich (R) comes close to Clinton’s numbers. Like Clinton, 47% say they’ll definitely vote against the former Speaker of the House.

Former Tennessee Senator Fred Thompson has the lowest number who will definitely vote against him at 30%. However, that’s probably a function of the fact that he hasn’t entered the campaign yet (only 22% say they’ll definitely vote for her).

A separate survey found that 17% say it’s Very Likely Hillary Clinton will be elected President in 2008. Nine percent (9%) say the same about Barack Obama. Seven percent (7%) say it’s Very Likely that Rudy Giuliani or John McCain will win the White House next year.

Rasmussen Reports provides continuously updated favorability ratings, ideological perceptions, and general election match-ups for all Republican and Democratic candidates. Results for the Generic Congressional Ballot are updated monthly. The President’s Job Approval ratings, which recently fell to all-time lows, are updated daily.

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Rasmussen Reports conducts ongoing surveys measuring the attitudes of Americans on the news of the day. Recent surveys have asked about immigration, the Pope’s comments on pro-choice politicians, the DC Madam, gun control, and the Supreme Court decision on partial-birth abortion. The latest updates can be found on the Rasmussen Reports home page.