Democratic presidential nominee Walter F. Mondale trails President Reagan by just 9 points nationally, according to a Harris Survey released today.

Harris reports that Reagan leads Mondale 53 to 44 percent, compared to 54 to 42 percent a week ago.

Mondale trailed Reagan by 13 points in the Harris Survey taken before the Oct. 7 debate in Louisville, which polls show Mondale won by a wide margin.

Harris reports that in the 11 biggest states, Reagan's lead has dropped from 11 points last week to 7 points currently -- 52-45 percent.

The survey of 1,393 "likely voters" was taken Oct. 12 to Oct. 14, after the debate between Vice President Bush and Geraldine A. Ferraro.

Mondale has lopped 15 points off Reagan's lead among Iowa voters, the Des Moines Sunday Register reported.

The latest Iowa Poll shows that Mondale, who was outdistanced by President Reagan 58 to 35 percent in September, now trails 51 to 43 percent among likely voters, the Sunday Register said.

The Iowa Poll also shows Rep. Tom Harkin (D-Iowa) leading incumbent Sen. Roger W. Jepsen (R-Iowa) by 5 points -- 46 to 41 percent. A month ago, the same poll showed Jepsen with a 9-point lead, which Harkin's campaign disputed at the time.

In Illinois, Sen. Charles H. Percy (R-Ill.) holds a 7-point lead -- 49 to 42 percent -- over Rep. Paul Simon (D-Ill.), according to the latest poll by the Chicago Sun-Times and Channel 5 News.