(Adds reaction from candidates)
By Edith Honan
NEW YORK, Oct 4 (Reuters) - The Connecticut U.S. Senate race is essentially tied one month before the November election as Democratic U.S. Representative Christopher Murphy tries to fend off former wrestling magnate Linda McMahon, a Quinnipiac University poll showed on Thursday.
McMahon, a former chief executive of World Wrestling Entertainment who lost a Senate race in the state two years ago, has battered Murphy with negative television ads since before the Aug. 14 primaries. The poll showed McMahon leading the three-term congressman 48 percent to 47 percent.
The majority of likely voters say they have seen McMahon's ads and most consider them effective, the poll found.
The poll could presage bad news for Democrats, who hope to hold on to their 51-47 advantage over Republicans in the 100-seat U.S. Senate. The chamber includes two independents, including U.S. Senator Joseph Lieberman of Connecticut, whose retirement after four terms in office created the open seat.
The candidates in the Democratic-leaning state have been locked in a dead heat since the primaries, setting up an unexpectedly competitive race in a state where President Barack Obama holds a double-digit lead over Republican Mitt Romney.
McMahon entered the race bruised from a defeat two years ago to Richard Blumenthal - then the state's attorney general - despite spending $50 million of her own money. But an aggressive ad campaign, which has sought both to reintroduce McMahon to voters and to weaken Murphy, has proven effective.
"McMahon has done a good job defining Murphy, who was not well known statewide, in a negative way," said Douglas Schwartz, director of the Quinnipiac poll. "McMahon's blanketing the airwaves with TV ads appears to be working."
The poll found 84 percent of voters have seen McMahon's campaign ads "often," compared to 64 percent who said the same of Murphy's ads. Two-thirds of voters say the McMahon ads are "very effective" or "somewhat effective," while about half said the same of Murphy's.
McMahon has portrayed Murphy as a no-show congressman who has repeatedly missed votes. Ads backing Murphy's campaign have made use of World Wrestling Entertainment videos that include portrayals of women that voters might see as crass.
In a statement responding to the poll, McMahon's campaign said voters "are wary of promoting a lifetime politician who failed to show up 75 percent of the time." McMahon has attacked Murphy for missing congressional committee meetings.
"Connecticut voters are clearly embracing Linda McMahon and her six-point jobs plan, the centerpiece of which is a middle-class tax cut that will save the average family $500 a month next year," said Corry Bliss, McMahon's campaign manager.
Eli Zupnick, a spokesman for the Murphy campaign, said McMahon is "spending tens of millions of dollars on lies, smears, and political attack ads in an attempt to distract voters from her strong support for right-wing Republican policies like ending Social Security, privatizing Medicare, and giving millionaires like herself another massive tax cut."
The statement described Murphy as a champion for middle-class families.
An analysis by the Wesleyan Media Project ranks the race the sixth most expensive Senate campaign in the country by broadcast television ad spending. During the last three weeks of September, $2.8 million was spent on McMahon ads, while less than $1 million was spent on Murphy ads, it said.
Overall, McMahon's supporters are more enthusiastic than her opponent's. Half of her backers describe themselves as "very enthusiastic," while 27 percent of Murphy's supporters said the same of themselves.
The survey of 1,696 likely voters was conducted over telephone landlines and cell phones from Sept. 28 to Oct. 2 and had a margin of error of plus or minus 2.4 percentage points. (Editing by Doina Chiacu)