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 even before the Aug 14 primaries.
The vast 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 onto their 51-47 advantage over Republicans in the 100-seat U.S. Senate. The chamber includes two Independents, including Connecticut Senator Joseph Lieberman, whose retirement after four terms in office created the open seat.
In Democratic-leaning Connecticut, the candidates have been locked in a dead heat since the August 14 primaries, setting up an unexpectedly competitive race in a state where President Barack Obama holds a commanding, double-digit lead against Republican challenger Mitt Romney.
McMahon entered the race still 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.
An analysis by the Wesleyan Media Project ranks the race the sixth most expensive Senate campaign in the country by broadcast TV. 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, according to the analysis.
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 and had a margin of error of plus or minus 2.4 percentage points.
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