WASHINGTON (Reuters) - Republicans swept hard-fought governor's races in Kentucky and Mississippi on Tuesday, expanding their base in the South as Americans cast votes in state and local elections that could set the political tone for 2004.

Kentucky voters elected Rep. Ernie Fletcher as governor, putting a Republican in the office for the first time in more than three decades. In Mississippi, longtime Washington insider Haley Barbour charged into the governor's mansion, turning out incumbent Democrat Ronnie Musgrove.

The double-barreled wins were good news for President Bush, who campaigned on behalf of both Republicans in races seen as possible bellwethers for 2004, when his re-election and control of Congress will be at stake.

Democrats held on to the mayor's office in Philadelphia as Mayor John Street won a bitter battle with Republican Sam Katz that turned on Street's claim he was the victim of a politically motivated FBI probe.

In Kentucky, Fletcher soundly beat Democratic state Attorney General Ben Chandler, grandson of former Kentucky governor, U.S. senator and baseball commissioner A.B. "Happy" Chandler, by a 10-point margin after a hard-nosed campaign that focused in part on Bush's economic policies.

"This really is a historic moment in this state," Fletcher said in becoming Kentucky's first Republican governor since 1971. "It really is a new era in Kentucky."

Chandler tried to tie Fletcher to mounting state job losses, repeatedly referring to the "Fletcher-Bush" economy. But Fletcher argued it was time for a change in the capital after a sex scandal engulfed Democratic Gov. Paul Patton, who admitted having an affair with a businesswoman he appointed to the state lottery board.

BARBOUR WINS MISSISSIPPI

In Mississippi, Musgrove lost the most expensive race in state history by an eight-point margin to Republican lobbyist and former national party chairman Haley Barbour, who blamed Musgrove for failing to attract new jobs or perk up a sour economy.

Barbour, who will be just the second Republican to govern Mississippi in the post-Civil War era, appeared to benefit from a heavy turnout among rural voters and the popularity of Bush in the southern state.

Strategists from both parties will pore over the results in Kentucky and Mississippi, both won by Bush in 2000, for possible trends heading into 2004. National Republicans were quick to point to the Kentucky results as a sign of Bush's strength.

"The Democrat strategy was negative attacks and tying Ernie Fletcher to President Bush and making this race a referendum on the president's economic policies," Republican National Committee Chairman Ed Gillespie said. "The Democrats had their referendum and got their answer."

A third governor's race, in Louisiana, will be decided in a Nov. 15 runoff to choose a successor to retiring Republican Mike Foster.

Republicans already hold governor's offices in Texas, Arkansas, Alabama, Georgia, South Carolina and Florida, building on what has been a reliably Republican electoral base in recent presidential elections.

In Philadelphia, Street easily won a re-election bid enlivened by the discovery of an FBI surveillance device during a routine sweep of the mayor's City Hall office.

Federal authorities said the device was part of a corruption probe of city government. Supporters of Street, a veteran black leader in a city where 80 percent of voters are Democrats, called the probe a Republican ploy to win the mayor's office and boost Bush's 2004 chances in Pennsylvania.

In other local elections, Republican Orlando Sanchez and Democratic former Deputy Energy Secretary Bill White headed for a runoff in Houston, the nation's fourth-largest city, after neither captured a majority of votes in a crowded field.