It's not certain just how much attack capability the US has at the moment. However, the nation may not be leaning solely on a well-timed media leak (provided this is accurate) to dissuade Russian hackers. The American government reportedly sent a "back channel" warning to the Russians, for one thing. Also, this wouldn't be the US' first rodeo -- it's launching online attacks against ISIS, and its intelligence agencies are no stranger to having access to whole countries' telecom networks. You underestimate the potential for retaliation at your own peril, in other words.
The more immediate fear is that Russia will try as much as it can without crossing the line. Guccifer 2.0 (who's allegedly a cover for Russian spies) and others could disseminate false leaks and bogus voting information in a bid to sway the election at the last minute. That may be difficult to stop. It's easy for anyone to whip up a temporary social network account that broadcasts misinformation for a few days, and it would be extremely aggressive to launch a hacking campaign as revenge for a fake scandal. Most likely, American leadership will be sitting on pins and needles until the final vote is cast... and possibly beyond that.