Republican candidates for the U.S. presidency are beginning a nationally televised debate that could have an impact on Tuesday's influential primary election in the northeastern state of New Hampshire.

Just before the New Hampshire debate got under way, an influential new opinion survey of nearly 700 likely New Hampshire voters showed Arizona senator John McCain leading the Republican field with 33 percent, well ahead of former Massachusetts Governor Mitt Romney at 27 percent. Former New York City Mayor Rudy Giuliani (14 percent) and Iowa caucus winner Mike Huckabee (11 percent) are trailing in third and fourth place in New Hampshire.

Democrats also are debating, in a separate nationally televised meeting beginning later. The new poll by CNN shows Senators Hillary Clinton of New York and Iowa winner Barack Obama of Illinois in a dead heat, sharing about two thirds of the primary vote (33 percent each). Former Senator John Edwards of North Carolina is well back in third place (with 20 percent).

Meanwhile, Republican Mitt Romney has won the Republican caucus in the sparsely-populated western state of Wyoming. The caucus was held Saturday but was largely overshadowed by events in New Hampshire.

The head of the U.S. Democratic National Committee, Howard Dean, said in his party's weekly radio address Saturday that the large turnout for the Iowa caucuses indicates Americans are committed to a change in the nation's political direction.

After New Hampshire's primary election Tuesday, other states are holding separate votes in the coming weeks. The state-by-state presidential nominating process leads to the Democratic and Republican parties' national conventions in August and September, setting the stage for the general election in November.