Two leading Democratic presidential candidates collected endorsements yesterday, as Sen. John Glenn (D-Ohio) ceremoniously welcomed the support of liberal Sen. Paul E. Tsongas (D-Mass.) while Walter F. Mondale countered with a long roster of public officials in three states.

In the ornate chambers of a Senate conference room, Glenn beamed contentedly in the klieg lights of the major television networks while Tsongas hailed him as the candidate who is most electable of them all.

Tsongas' endorsement triggered speculation as to whether he is blazing a trail for liberal Democrats to put their convictions on "hold" in the name of defeating President Reagan.

There was no immediate stampede, however.

Tsongas confirmed that he had unsuccessfully tried to enlist two liberal Senate colleagues, Bill Bradley of New Jersey and Christopher Dodd of Connecticut.

Mondale campaign officials responded yesterday by announcing endorsements by dozens of major public officials in Iowa, Michigan and California, endorsements they said had long been scheduled for this week as a prelude to the Wisconsin Democratic convention this weekend.

The Iowa endorsement came in the form of a 1,000-member Mondale "steering committee," headed by state Attorney General Tom Miller, Rep. Tom Harkin and former state Democratic chairman Edward Campbell.

Mondale hoped to embarrass California's presidential contender, Sen. Alan Cranston, by announcing the support of a number of Democratic mayors, including San Francisco's Dianne Feinstein and Oakland's Lionel Wilson.

In Michigan, Mondale announced a blitz of endorsements, including Gov. James Blanchard, Detroit Mayor Coleman Young, the secretary of state, the attorney general, 13 other statewide officials, 50 county chairmen, the speaker of the state house, 30 state house members, the state senate majority leader and eight state senators.

Today it is Glenn's move again. In Missouri he will counter with former Democratic national treasurer Charles Curry plus his own state house and senate luminaries.

Tsongas and Glenn have been on opposite sides of several major issues, including Reagan's tax cut plan and the B1 bomber.

"This is a painful decision given my personal and ideological closeness to the other candidates, all of whom I deeply respect," Tsongas said in his prepared statement. When asked why he didn't think one of the more liberal candidates could be elected, he replied:

"I think there is a time and a place for ideology..., and I don't believe that mine, and those who share my ideology, can succeed in 1984....You need a candidate who can appeal to the heart, if you will, of the American people--and still lead them in a different direction."