If he's so far ahead in his race for reelection, Sen. Richard G. Lugar was asked at a news conference here, why has he gone on the air with slashing attacks on his underdog Democratic challenger, Rep. Floyd J. Fithian?

Lugar smiled sweetly and responded, "All things considered, I'd rather win."

Just as Republicans two years ago rode a late-breaking tide for President Reagan to capture control of the Senate, Democrats are hoping this year that a wave of economic discontent will wash out Republicans, possibly enough for the Democrats to regain a Senate majority.

It is a measure of the uncertainty less than a week before the Nov. 2 elections that even a front-runner like Lugar, who was elected to the Senate six years ago by the biggest margin in Indiana history, is punching away at his opponent, putting some distance between himself and Reagan and generally leaving nothing to chance.

In Democratic as well as Republican polls, Lugar, a conservative running in a conservative state, has a comfortable lead. The Republican Senatorial Campaign Committee hasn't bothered to do tracking polls on him.

He can outspend Fithian by 4 or 5 to 1. Indiana Republicans have one of the best political organizations in the country; the Democratic organization is chaotic by comparison.

While polls indicate that Fithian has yet to scare the daylights out of Indiana voters with his doom-and-gloom campaigning on economic woes, even Lugar concedes that he has narrowed the gap.

Last week Lugar figured he was ahead by 15 to 20 percentage points.

Fithian, however, contended that Lugar was leading by only 8 to 12 points, and took cheer from Democratic polling that indicated he'd whittled Lugar's share from 64 percent to less than 54 percent, with the relatively small number of undecided voters leaning in his direction.

Lugar claims he leads even among the unemployed, although Fithian's polls reportedly show a 2-to-1 lead for Fithian among families of the unemployed.

So why is Lugar running radio commercials that savage Fithian's attendance record in the House and stop barely short of branding him a liar in their debate over Social Security? Why does Lugar make a point of his sponsorship of a housing aid bill that Reagan vetoed, and why does he use the word "compassion" at a least a half dozen times in a single speech?

Like many other Republican incumbents, Lugar is buying extra insurance against a possible surge for the Democrats in reaction to the economy, Reagan's handling of it and Lugar's strong record of support for the Reagan economic program in Congress.

If he winds up with more votes than needed, so be it. A big victory would help Lugar in his plan to challenge Sen. Bob Packwood of Oregon as chairman of the Republican Senatorial Campaign Committee.

Lugar sums up his main problem in his campaign slogan, "A Good Man for Tough Times."

With an economy dominated by steel, autos and farming, Indiana has been reeling for three years under double-digit unemployment rates, and the blows keep coming.

Unemployment is pushing 20 percent in the steel towns along Lake Michigan. In Anderson, an auto town south of here, the current rate of 13 percent looks good only by comparison with the 22 percent level of a few months ago. Statewide, the unemployment rate is 11.3 percent, down from nearly 13 percent, but higher than the national average of 10.1 percent.

A Purdue professor of diplomatic history, Fithian, 53, was elected to Congress from a heavily Republican district in the post-Watergate year of 1974. Having been redistricted out of his House seat, he's pinning his Senate hopes on a backlash against Reaganomics.

An energetic, voluble campaigner, Fithian reaches out to labor, minorities and other traditional Democratic voters with an apocalypse-now urgency that has prompted Lugar aides to call him "Dr. Gloom." "I'm fearful for this country . . . .I don't think we have a whole lot of time left," he told a group of United Auto Workers officials one morning last week. "Sure the stock market is up . . . just like it was in 1929." He made the remark before the stock market on Monday suffered its second-largest one-day loss since Oct. 28, 1929, the date generally used to mark the start of the Great Depression.

Polls mean a lot to Fithian because he has to show the race is winnable to raise enough money to stay in it. He hustled out his latest poll results to potential givers as soon as they were in hand and claimed that they could give enough, along with a $50,000 loan from the state Democratic Party, to buy television advertising for the campaign's final days.

Lugar, 50, wears the GOP label proudly but not too conspicuously, prompting jibes from Fithian, who wears his Democratic trademark as prominently as possible.

Even with a Democratic tide, Lugar can count on a powerful precinct-to-statehouse Republican organization for a massive get-out-the-vote drive.

Fithian, by contrast, is relying heavily on unions to turn out his vote. "We figured we'd have money for organization or for media, not both," he said as he drove the same Indianapolis-to-Anderson-to-Fort Wayne route that Lugar would fly the following day in a chartered twin-engine plane.