O'Neill comes out swinging.

Democratic House speaker Thomas P. (Tip) O'Neill Jr. yesterday tried to infuse spirit into his party's presidential campaign, charging that Walter F. Mondale has been ''too much of a gentleman'' and should begin to counter Republican punches.

Former Vice-President Mondale should ''come out slugging, and come out fighting,'' said the white-haired Speaker as he opened the House for the year's final scheduled session.

''They've been beating ... Mondale, and Mondale hasn't been responding,'' complained O'Neill. ''He's got to hit the issues of war and peace, the deficit, fairness, and the lack of knowledge of the President.''

The frank public message comes in the wake of polls showing Mondale running far behind the President. But it is probably only a foretaste of the earful that the Democratic nominee can expect when he arrives today for meetings on both sides of Capitol Hill.

O'Neill held that, despite public-opinion samplings, ''I think Mondale has an excellent chance of winning. I think it's much closer than in the polls.''

The Speaker, who has spent much of the last three years as the President's prime antagonist, renewed his blasts of the Reagan administration. He focused on the traditional Republican standby - balancing the budget - while raising the specter that a Reagan administration might cut social-security benefits.

''I will bring a balanced budget to the floor 48 hours after (Reagan) submits one,'' O'Neill challenged. ''The American people have a right to know before the election what the President has up his sleeve.''

''President Reagan is trying to skip the campaign,'' complained O'Neill, by limiting debates. ''He decrees an end to press conferences as if he were an emperor.''

While flaying the President, the House Speaker also said that ''I admit the popularity of the man.'' But he maintained that the Reagan popularity will not spill over into congressional races this year. The voters ''want a balance out there,'' he said, in predicting that the Democrats would gain a handful of seats.

Meanwhile, a top official in the Democratic National Committee also conceded the difficulties facing the campaign of Mondale and vice-presidential nominee Geraldine A. Ferraro.

At a breakfast with reporters, DNC political director Ann Lewis agreed that crowds had been sparse for the official start of the presidential race this week , that the Mondale campaign had been ''immobilized'' in August by the controversy over his running-mate's personal finances, and that if the election were held next week, Ronald Reagan would win.

She said that her party must mount an ''aggressive'' campaign countrywide, even in Reagan strongholds like California, to gain momentum in the next two months.

Echoing a persistent Democratic frustration, Mrs. Lewis said that priority No. 1 is to ''establish a connection between Ronald Reagan'' who is personally popular, and Ronald Reagan's views - such as on the war-peace issue and religion - which are not. Moreover, she said that the contest must be waged over the future, not as a rerun of the 1980 campaign.

The chief strategy for keeping the focus on the future so far has been reminding voters of the growing federal deficit, and the need for raising taxes to reduce it.

Mrs. Lewis said that Mondale's approach to taxes is ''the politically right thing to do'' because the voters ''know out there that we are headed for economic problems.''

The tactic is risky, she conceded. ''I would rather run a campaign as Santa Claus,'' she said, adding that ''Santa Claus'' has been running the government and ''the bill for the Christmas shopping is about to come.''