Sen. Albert Gore Jr. (D-Tenn.) has opened up a new front in his war to establish himself as the only "strength" Democrat running for president. At a debate Friday night in Sioux Falls, S.D., he asked former senator Gart Hart whether Hart believes the U.S. deployment of medium-range missiles in Europe, a policy conceived by President Jimmy Carter and carried out by President Reagan, was responsible for the Soviets' agreeing to the INF Treaty.

For Reagan to take credit for the INF, Hart replied, "is like the rooster taking credit for the dawn." He said he believes the Soviets had wanted to negotiate away their medium-range missiles in Europe years ago, and their decision had nothing to do with deployment of the medium-range and short-range missiles. The other Democrats at the debate did not disagree, all saying that the Soviets' foundering economy drove them to the table.

"I find {this} difficult to believe," Gore said, savoring a distinction he is sure to draw as he campaigns through the South. "Do you really think the Soviets just said to themselves, 'We're feeling some economic pressure: Let's remove our SS20s' ?"

Gore did not escape unscathed. On another matter, former Arizona governor Bruce Babbitt asked him: "You came to Iowa, set up your tent, read the polls, folded up your tent and left. What does that say about leadership and persistence?"

Gore said he found the Iowa process full of distortions. "If you are going to be a national candidate," Massachusetts Gov. Michael S. Dukakis piled on, "you have to campaign everywhere."

Meanwhile, Sen. Paul Simon (D-Ill.) attacked Rep. Richard A. Gephardt (D-Mo.) for voting for the Reagan tax cut, the grain embargo and the 1985 farm bill and against an increase in the minimum wage. Gephardt shelled Simon for backing a sub-minimum wage, the line-item veto, the balanced budget amendment and the Senate version of the Gramm-Rudman-Hollings budget bill.