The addition of former Tennessee Senator Fred Thompson (R) to the list of candidates shakes up the race for the GOP Presidential nomination.

Former New York City Mayor Rudy Giuliani (R) remains on top, but his support dips below the 30% mark for the first time in seven weeks. With Thompson in the mix, Giuliani’s support tumbles to 26%, down nine points from a week ago. That’s the lowest level of support measured for Giuliani in any Rasmussen Reports poll this year.

Support for Arizona Senator John McCain remains steady at 16%, but McCain’s hold on second place is threatened by Thompson. The movie star turned Senator turned TV star weighs in with 14% support among those likely to vote in a GOP primary. Among Very Conservative voters, Giuliani attracts 20% support followed closely by Thompson at 19%, former House Speaker Newt Gingrich at 18% and McCain at 14%.

Overall Gingrich is preferred by 11% while 8% would rather see former Massachusetts Governor Mitt Romney as the GOP nominee. Former Arkansas Governor Mike Huckabee reaches 3% and no other candidate attracts support from more than 1% of likely voters. Rasmussen Reports releases updated polling data on the Republican nominating contest every Tuesday. Results for the Democrats are updated on Mondays.

In general election match-ups Thompson is tied with New York Senator Hillary Clinton (D) but trailing Illinois Senator Barack Obama (D).

Giuliani leads all Democrats but polling released on Monday showed that Obama has pulled to within a single point of “America’s Mayor.” Giuliani holds larger leads over Clinton, former North Carolina Senator John Edwards, New Mexico Governor Bill Richardson, and Senators Chris Dodd and Joe Biden.

Rasmussen Reports continuously updates favorability ratings and general election match-ups for all Democratic and Republican candidates. We also maintain ratings for Members of Congress, Other Political Figures, and Journalists.

Rasmussen Reports releases updated polling data on the Republican nominating contest every Tuesday. Results for the Democrats are updated on Mondays.

The current survey is based upon national telephone interviews with 590 Likely Republican Primary Voters conducted March 26-29, 2007. The margin of sampling error is +/- 4 percentage points with a 95% level of confidence.