President Barack Obama may be feeling the heat of a summer of challenges, as well as angst about what's happening in Iran. His poll numbers are inching downward and now are hitting new lows.

A three-day average of daily Gallup Poll surveys finds Obama's job approval hitting 57 percent, his lowest since taking office. He'd dipped below 60 percent in Gallup's polling four times before this week, but never hit 57 percent.

The Gallup three-day average also found 35 percent of American adults disapproving of the way Obama's handling his job.

At the same time, a new Rasmussen Poll finds Obama's job approval among likely voters at 54 percent Monday - up a point from a low of 53 percent marked over the weekend.

The average approval rating of six recent public polls, compiled by the website realclearpolitics.com, is now 58.7 percent. The average disapproval is 33.7 percent.

As we noted a few weeks ago, Obama's entering a tough new phase of his presidency, more difficult than his first 100 days. Among the summertime challenges: pushing a health care overhaul that could depend on broad tax increases and now has a sky-high price tag courtesy of the Congressional Budget Office. 

Also, the Iran election and criticisms that Obama didn't do enough to help or encourage protesters,  rising fears that North Korea will become a nuclear power, rising gas prices and a continuing recession, to name a few.

As a snapshot, a nearly 59 percent average approval rating is still pretty good, and suggests Obama still has plenty of political capital to use, at least in the domestic political fights. 

But if there is a downward trend, and he continues to lose support, that could make Democrats facing re-election next year increasingly nervous about following his lead. And that would make his job all the more difficult.