In a hypothetical race between Chicago Teachers Union (CTU) President Karen Lewis and Chicago Mayor Rahm Emanuel, Lewis leads the sitting mayor, a new poll shows.

Lewis garnered 45 percent of the respondents' support compared to Emanuel's 36 percent showing in the automated telephone poll of 1,037 likely voters on July 9. Eighteen percent of the poll's respondents were undecided. 

Lewis is not a declared 2015 mayoral candidate, but she is "seriously thinking" about challenging Emanuel.

“I’m a little sick of the mayor and I don’t see anyone stepping up,” Lewis said last month. “I am seriously thinking about it.”

The margin of error for the automated telephone poll, conducted by We Ask America for the Chicago Sun-Times, is plus or minus 3.04 percentage points.

Among African-American voters, Lewis was the top choice at 51 percent to Emanuel's 33 percent. Hispanic respondents of the poll also favored Lewis over Emanuel, while Asian and white voters preferred Emanuel. 

“Wow,” Lewis told the newspaper in response to the poll's results. “Well, first of all, I’m sitting here stunned.”

The poll also look at a hypothetical race between Cook County Board President Toni Preckwinkle and Emanuel. Preckwinkle, a former city alderman, leads Emanuel 55 percent to 31 percent in the poll. Preckwinkle says she's running for re-election, but hasn't fully ruled out a mayoral run.

Pete Giangreco, an Emanuel campaign spokesman, dismissed the poll results as "laughable."

“While the Mayor is focused on helping every neighborhood grow stronger, the campaign finds these inaccurate robo polls entirely laughable,” he told the newspaper. 

Meanwhile, in the mayoral competition between Emanuel and former Chicago 9th Ward Alderman Robert Shaw, who announced plans for a 2015 mayoral bid, Emanuel leads Shaw 47 percent to 29 percent in the poll.

In a hypothetical race between Emanuel and Ald. Bob Fioretti (2nd), who is not a declared 2015 mayoral candidate, the sitting mayor was the top pick at 45 percent to Fioretti's 30 percent.

Click through for more on the poll's results.