DES MOINES, JAN. 27 -- After the campaign staff of Sen. Paul Simon (D-Ill.) planted the rumor, aides of fellow presidential candidate Rep. Richard A. Gephardt (D-Mo.) spread the word today that Simon was about to air television commercials critical of Gephardt.

It turned out not to be true, but the Gephardt staff's exertions guaranteed a large turnout at a news conference here unveiling two Simon commercials, including one in which the senator talks about why he wears bow ties. It's "my declaration of independence," Simon says. "I'm going to be my own person. You have to take me for what I am."

The Gephardt campaign, by raising the specter of negative advertising, gave the Simon people a chance to portray their candidate as a principled politician who would never think of running a negative ad.

"I'm glad I'm doing such a good job for Paul Simon," quipped a chagrined Laura Nichols, Gephardt's Iowa press secretary.

"The Gephardt campaign is in a paranoiac furor that we were about to unload on them," gloated Simon's media consultant, David Axelrod.

And so go the television ad wars in the increasingly combative days leading up to the Feb. 8 Iowa precinct caucuses.

The battle reflects the heightened aggressiveness of the candidates at televised debates and other forums. Gephardt has attacked Simon's economic program as "Reaganomics in a bow tie" and Simon has chided Gephardt for supporting President Reagan's tax cut and implied that Gephardt has changed his positions for political expediency.

The attacks come as polls show that Gephardt, who was falling behind last November, pulling even with Simon at the front of the pack, with Massachusetts Gov. Michael S. Dukakis nipping at their heels. Gephardt's rise, say aides in both camps, came largely because of a $500,000 blitz of television commercials about "saving the family farm" and combating unfair foreign trade practices. Simon is estimated to have spent $400,000 on television here over a longer time.

Today at Simon's Iowa campaign headquarters, Axelrod said the new commercials, to begin airing Thursday, are calculated "to make people feel comfortable with Simon as a person." In the second commercial, Iowans, shown in still photographs, praise Simon as being "tied to the soil," "motivated by a desire to see everyone have an equal opportunity" and a "traditionalist."

The commercial is filmed in black and white, Axelrod said, because "we're in a stage of the campaign where you need to do things that will stand out."

Standing out, so far, does not call for negative advertising. Within the Simon camp, he said, "there was no real argument in favor of doing a sharp, negative television commercial . . . . It is very dangerous to go negative in a multi-candidate race."

Gephardt has two spots currently on the air, synthesizing his themes of farm and trade, aimed at people who have suffered in an Iowa economy with troubled farms and a shrinking industrial base. "I've been all across this country and I've looked at the eyes of Americans, and I don't think you're ready to give up yourselves," Gephardt says in a spot entitled "People." "We have to stand up for our jobs, our farmers, our seniors. If we don't, who will? . . . Family farmers don't want any more sympathy," Gephardt says in the other spot.

"They need a president who defends them -- not some theory of one-way free trade," he said.

Axelrod attributed the success of Gephardt's commercials to "a message that was simple, compelling . . . . They've achieved total coordination between paid and unpaid media so that every time Dick speaks, every time the candidates debate, he's delivering his campaign commercial."

Dukakis, meanwhile, is airing ads designed to "show that he has passion," said his media consultant, Ken Swope. One commercial shows homeless people in Washington as Dukakis says that "decent and affordable housing ought to be the birthright of every American."

In another, "little American miracles" -- that is babies -- are shown crawling in front of an American flag as an announcer says Dukakis can safeguard their future because he "took a dying economy and helped turn it into a booming international competitor."

Dukakis, in debates at least, has gone on the attack, saying that the candidates from Congress bear responsibility for the federal deficit. "What have you folks been doing for the last seven years?" he demanded of Gephardt and Simon at the last debate.