Slideshow ( 2 images )

LOS ANGELES (Reuters) - California Democratic Senator Barbara Boxer has extended her narrow lead over Republican challenger Carly Fiorina in her closely watched bid for re-election in November, according to a poll released on Friday.

Boxer, facing the toughest electoral fight of her Senate career, led Fiorina, a former Hewlett-Packard CEO, by 47 percent to 41 percent in the Field Poll.

That marked a slight gain from the Field Poll conducted in July, which showed the three-term senator leading Fiorina, making her first run for public office, by 47 percent to 44 percent.

Republicans, who see a chance to regain majorities in the U.S. Senate and House of Representatives in the November 2 elections, would love to defeat Boxer, an outspoken liberal and strong supporter of President Barack Obama’s agenda.

Many political analysts have called the Boxer-Fiorina race a tossup, saying Boxer has been hurt by high negative ratings and faces a well-financed candidate who can trumpet her status as a Washington outsider and self-made Silicon Valley businesswoman.

Fiorina also faces obstacles, including running as a conservative in California, which has 2 million more registered Democrats than Republicans.

Field Poll directors said the poll results showed the 2010 election was more about incumbent Boxer than Fiorina.

According to the poll, 65 percent of those who supported Fiorina said their choice was more of a vote against Boxer than affirmation of the former Silicon Valley executive.

The poll surveyed 599 registered California voters from September 14 to 21 and had a margin of error of plus or minus 4.1 percentage points.