Even when he was roasting Democrats every day as Gerald Ford's 1976 running-mate, Sen. Bob Dole enjoyed an underground reputation for bonhomie and wit. Its sudden discovery by a larger national audience commenced only recently.

He proved to be the only major congressional figure who declined to be intimidated by the unpopularity of revenue-raising. While the House gaped and wrung its hands, Dole engineered a new tax bill, sold Ronald Reagan on it, and pushed it through the Senate.

Basking in the afterglow, Dole doesn't shrink from the agreeable suggestion that Wall Street's recent bull market had at least a bit to do with his tax bill. On a recent New York trip he was lionized at the stock exchanges, where brokers told him he had renewed their faith that Congress can, at least occasionally, do something that needs doing.

Now, crashing a bit further into what is generally regarded as a man-killing political thicket, Dole has taken up Social Security reform. In recent letters to the president and to congressional leaders, he proposed a special post-election session to undertake repairs to Social Security.

The subject is so unpopular, Dole says, that at least two members of his Finance Committee boycott meetings at which Social Security is on the agenda. When congressmen discuss Social Security at all, they do it in the Alice-in-Wonderland spirit of those who can believe six impossible things before breakfast.

But Dole, a hard-headed student of elementary arithmetic, knows that a collapse of Social Security payments is ominously possible -- soon. His figures are vivid and disturbing. The Old Age and Survivors Benefit trust fund is being depleted at the rate of $30,000 a minute. The fund is down to less than three months' reserves. Payments are sustained only by interfund borrowing, an expedient that Congress has authorized only through December.

Unfortunately, there is no painless fix. And the Social Security issue has been so scandalously politicized that Congress is almost afraid to touch it.

Dole wants Congress to seize what he calls "a window of opportunity" between the fall elections and the January seating of a new Congress to put Social Security on a sounder actuarial footing. By mid- or late November, Congress should have at hand the proposals of a commission on Social Security financing now at work under the chairmanship of economist Alan Greenspan. If that body can agree on some reform proposals, Dole thinks, Congress should be able to act on them before political temperatures begin to rise again.

Having passed the tax bill that doubters thought impossible, Dole seems to be in an in-for-a-penny, in- for-a-pound mood. If he succeeds in rescuing Social Security from insolvency, this city will be forced to suspect Bob Dole of statesmanship as well as wit.