Chicago voters have grown sharply dissatisfied with Mayor Rahm Emanuel's job performance and have signaled a willingness to support his most prominent potential challenger, a new Chicago Tribune poll has found.

Facing concerns over crime, education and the economy, Emanuel now holds a 35 percent job approval rating — down from 50 percent a little more than a year ago. In addition, more than half of city voters now disapprove of how Emanuel has handled his first term, up from 40 percent.

During the past year, the mayor's approval rating has dropped across all major racial, income, age and gender lines. Perhaps most troubling to Emanuel's re-election: crumbling support among white voters and an accelerating decline of support among African-American ones.

The discontent has provided an opening to Chicago Teachers Union President Karen Lewis should she decide to challenge Emanuel next year.

In a head-to-head matchup, the survey found Lewis with 43 percent backing to 39 percent for Emanuel, with 14 percent undecided. Lewis' advantage is just outside the 3.5 percentage point error margin for the poll, consisting of cellphone and landline interviews with 800 registered city voters from Aug. 6 to Tuesday.

Although those numbers are not a welcome sight for Emanuel as he seeks a second term, the mayor still has no significant declared opposition and has stockpiled $8.3 million in his campaign fund during an era of contribution limits. That would allow him to rebuild his image while defining eventual opponents to voters before they can define themselves.

Such a reality could chip away at Lewis' support should she run. So far, Emanuel's most prominent potential challenger has put her chances of getting into the race at "50-50."

But the poll also found that even a mostly unknown potential challenger — 2nd Ward Ald. Bob Fioretti — has gained surprising traction.

While 4 out of 5 voters had no opinion of Fioretti, 1 out of 4 said they'd back Fioretti over Emanuel — a sign that there's a sizable contingent of anyone-but-Emanuel voters. The mayor had 43 percent in the hypothetical matchup with Fioretti, while 23 percent were undecided.

For any politician aiming to hold onto an office, a job approval rating of at least 50 percent is key. Anything below that reflects an elected official in trouble. By that standard, the poll shows Emanuel is vulnerable. Even the white voters who have long formed a base of support for the North Side politician are increasingly disenchanted.

In a May 2013 Tribune poll, 59 percent of white voters approved of the job Emanuel was doing. Now 44 percent approve. And those who disapproved of his job performance rose from 32 percent last year to 46 percent today. All told, more white voters disapprove of Emanuel's performance than approve — a first in a Tribune poll. APC Research, which conducted the poll, is a nonpartisan market survey firm long used in Tribune polling.

Emanuel's backing among African-American voters continues to tumble. Nearly 6 in 10 black voters disapprove, while only about 1 in 4 approve of the mayor's performance. In May 2013, 48 percent of black voters disapproved of the mayor's job performance while 40 percent approved.

Even a majority of Hispanics — a small but growing demographic in city voting — found Emanuel's job performance lacking, a reversal from last year.

In recent weeks, Emanuel has deflected re-election talk, even as his camp has shown a willingness to fire warning shots at would-be challengers to give them an idea of the pitched battle that lies ahead should they run for mayor. When Cook County Board President Toni Preckwinkle was considering a run earlier this year, Emanuel's top political consultant shopped potential negative stories about her to reporters. Last month, Preckwinkle announced she wouldn't run.

"The way I look at it, there's a time for polling and there's a time for politics," Emanuel said last month. "My time now is focused on the priorities and the progress on priorities for the future of the city of Chicago."

Emanuel has good reason not to talk about his re-election or potential opponents — he doesn't have to yet.

When speculation was rampant that Preckwinkle might run against him, Emanuel would not engage her directly, instead choosing to say repeatedly that she had told him she wouldn't run. With the focus now shifted to Lewis, Emanuel continues to exercise the same discipline by not referring to the union leader by name or discussing a possible matchup.

No issue drew greater protest in the last year than Emanuel's decision to close nearly 50 neighborhood schools he argued were underenrolled and straining the district's finances. Most of the school closings occurred on the South and West sides, affecting many African-American families.

Dissatisfaction over Emanuel's handling of the city's school system was evident among the 19 percent of the survey's sample with children in Chicago Public Schools and the nearly one-third who came from union households. More than 6 in 10 CPS parents and union household members disapproved of Emanuel's job performance.

That dynamic also emerged when it came to choosing between Emanuel and Lewis, the poll found. Among CPS parents, 57 percent backed Lewis, who led an eight-day teacher strike during Emanuel's first year in office, while 27 percent supported Emanuel. Similarly, 56 percent of union household members backed Lewis, who has headed the CTU for four years, while 31 percent backed Emanuel's re-election.