With polls showing that Americans overwhelmingly disapprove of the breakup of the national Bell telephone system, it was probably inevitable that some place, some time, some politicians would try to milk the issue for electoral gain.

The place is Colorado, and the occasion is the state's tight 1986 Senate race. The state Republican Party has mounted a hard-hitting ad campaign charging that Rep. Timothy E. Wirth, the Democratic Senate candidate here, "led the fight to break up the best telephone system in the world."

Pointing out that Wirth proposed House legislation designed to revamp the Bell system before the court-ordered American Telephone & Telegraph divestiture, the GOP has pushed that point all spring in the mass media, in mailings and in a flood of editorial columns.

So far, however, the telephone campaign seems to be ringing a wrong number for the Republicans.

Newspapers across the state -- including many conservative, rural papers that rarely express sympathy for the liberal Wirth -- have denounced the GOP campaign as false and unfair. In a typical comment, the Grand Junction Daily Sentinel called the GOP ads an "inaccurate, unfair smear campaign."

Further, the GOP has managed to alienate both management and labor at Mountain Bell Telephone Co., the state's largest private employer. Mountain Bell -- miffed at suggestions in the GOP ads that telephone service is worse than it used to be -- demanded that the Republicans stop their ad campaign. The Communications Workers of America called the GOP ads an "insult" to telephone workers.

Finally, the campaign seems to have been a flop with voters. A Denver Post poll last week showed that more than half the Coloradans who have heard the GOP ads don't believe them. Of Republicans polled, 33 percent described the anti-Wirth ads as untruthful.

As a result, many political analysts here say the campaign on the telephone issue has turned into a net plus for Wirth. "If they push this one hard and can't make the charges stick, you'd think it would be tougher for the Republicans to go after Wirth on the next issue," said David Greenberg, a Denver political consultant.

The Denver Post poll, taken in late May, showed Wirth leading the Republican Senate nominee, Rep. Ken Kramer, by 42 percent to 33 percent. Six months earlier, the poll had shown Kramer and Wirth running even.

The idea of blaming Wirth for the unpopular AT&T breakup originated in 1984, when Republicans used the charge with some effect during Wirth's surprisingly close race that year for reelection to the House. "Every time we brought it up, Wirth lost his cool," said Howard (Bo) Calloway, the state GOP chairman. "We figured if it gets him that flustered, it's a good issue for us."

This year, when Wirth announced for the Senate seat, Calloway laid out an extensive campaign. In letters, speeches and opinion columns, Republicans charged that Wirth used his position as chairman of the House telecommunications subcommittee to fight the national Bell system.

The party ran tough radio ads full of gibes such as: "Tim Wirth's bright ideas about breaking up the phone company have led to higher rates and lots of confusion."

But Wirth fired back with reams of documentation aimed at showing that divestiture was not his doing. He noted that the Justice Department suit was filed before he went to Congress, and that the eventual breakup was the result of a settlement between AT&T and the Reagan administration.

The polls and editorials here suggest that the AT&T breakup is still unpopular, but that Wirth has succeeded in deflecting the blame for it. Consequently, the GOP seems likely to hang up the phone.

Kramer, who was chosen Saturday to be Wirth's Republican opponent, has been shying from the AT&T issue already. His aides say there will be little, if anything, said about the telephone breakup during the fall campaign.