It's campaign countdown time for Glenn

John Glenn has decided it's time to ''go for it.'' With just 100 days left before the first voting in Election '84, Senator Glenn has begun an all-out drive for support in the early presidential primary and caucus states.

The effort may be coming none too soon. A dozen leading political pollsters and analysts interviewed over the past week said unanimously that front-running Walter F. Mondale now stands on the brink of locking up the Democratic presidential nomination.

Glenn, who is No. 2 in the national polls, is hurrying to slow the Mondale momentum. He has shaken up his campaign staff in recent days and has rushed money and talent into Iowa, New Hampshire, and four or five other early-voting states.

This week he also fired a one-two salvo at Mr. Mondale. First, in the toughest speech of his campaign, he blasted Mondale for what he called ''a fundamental lack of support for an adequate defense.''

Wednesday night he went after Mondale's ''do everything'' economic promises to teachers, labor unions, and others - promises that he charges would cost the federal Treasury $89 billion and make it impossible for Mondale to reduce the government's $200 billion-a-year deficits.

The attacks on Mondale have several purposes.

Glenn's private polls have determined that the Ohio senator's positions on the issues are ''much more attractive'' to Democratic voters than Mondale's, a Glenn strategist says. Approximately 40 percent of the party's voters back Glenn's policies on specific issues, about 30 percent back Mondale's, and the remaining 30 percent fall somewhere in between, he says.

''This means that if we can demonstrate, first, that John Glenn is qualified to be president, and, second, that Glenn differs from Mondale on the issues, we can be very confident of the outcome,'' says a Glenn planner.

This week's speeches, showing how Glenn differs point by point from Mondale, started that process, and more attacks on Mondale will quickly follow.

The new Glenn strategy comes in the wake of what experts call a ''phenomenal'' month for Mondale. In a 30-day period, Mondale won the backing of the AFL-CIO, the National Education Association, and key political figures like Mayor Tom Bradley of Los Angeles.

The campaign gathered so much momentum that one rival campaign manager called it ''awesome.''

The results showed up right away in the polls. The most dramatic gain in Mondale's strength was reported by the Washington Post/ABC poll. It showed that Mondale widened his margin over Glenn among Democratic voters, gaining 48 percent to Glenn's 20 percent (up from 41 to 28 in late September).

Other polls, such as the latest Garth Analysis, don't show the margin quite that wide. Garth, for example, found a spread of 34 to 21 in Mondale's favor. But Garth also found the spread getting wider - a trend that Glenn must halt quickly if he is to remain a credible opponent.

While Mondale's campaign may be awesome, analysts say that it is still too early to consider the race over. The Mondale team has a number of worries, any one of which could trip the front--runner. Those concerns come under a number of headings:

The liberals. At one time, Mondale might have expected to get most of the party's large liberal vote. But so many liberal candidates are now in the race that the liberal vote will probably be split at least five ways. The black vote, which is heavily liberal, illustrates the point. Mondale once could have counted on two-thirds to three-fourths of the black vote in the primaries. But Jesse L. Jackson's newly launched campaign has lighted a fire in the black community, and about half of Mondale's black vote is expected to switch to Jackson.

Timing. The first primary state is New Hampshire, which has shown a strong affinity for conservative and moderate politicians. While Mondale is well organized there, Glenn still hopes for an upset. Even if he gets over that hurdle, Mondale must get past the ''Super Tuesday'' primaries in the South (Georgia, Alabama, Florida), where a pro-defense candidate like Glenn should do well.

''Soft'' support. Mondale may be ahead. But his support, pollsters say, is squishy. ''The only people thinking about the election right now are about 2, 000 party activists and the press,'' laughs one poll taker. As few as 10 percent of the voters have firmly decided on a candidate, analysts say. This means Mondale's support could drop precipitously if he makes a gaffe, or if he loses a primary, or if another candidate comes on strongly.

Lack of a majority. Mondale's team says everything in the campaign has gone according to plan so far. If that is so, why, according to some polls, is his strength still only in the 30 to 35 percent range in the party? Has Mondale peaked, as Glenn supporters claim? As other contenders begin campaign advertising on TV and step up their attacks on Mondale, will the former vice-president find the going tougher?

Atop all of this, both Mondale and Glenn have another worry: President Reagan is getting stronger and stronger. Recent upticks in the economy and the successful mission on Grenada have sharply boosted his ratings.

The latest Garth study, comparing Reagan to Mondale, finds the President widening his margins over the front-running Democrat. By a spread of 66 to 19, American voters perceive Reagan as a stronger leader.