RICHMOND, AUG. 29 -- Moments before going onstage to address a meeting of southern legislators last week, Mississippi state Rep. Charles Capps turned to Charles S. Robb and uttered the words the former Virginia governor has heard countless times this year:

"I said to him, 'Governor, I think you are the only electable Democrat we have, the only one with the national credentials who could win' " the presidency in 1988, Capps recalled. Robb demurred, but consoled him by saying that conservative Democratic Sen. Sam Nunn of Georgia would soon end months of speculation and announce his own bid for the party's nomination, Capps added.

However, two days ago, Nunn rejected a run for the White House, disappointing not only Robb, a friend and ideological ally, but conservative and moderate Democrats throughout the South. At the same time, say Virginia Democrats and some party leaders in other southern states, Nunn's decision turned up the pressure a notch for Robb to define his own immediate political plans.

"I'm going to have to make some decisions," Robb said today in a telephone interview. "The little niche that has been extended to me, well beyond the life of most former state officials, does not last forever."

Although he insisted that Nunn's decision "does not affect my plans" immediately, the fact remains that Democrats here and elsewhere are more eager than ever for Robb to challenge first-term Sen. Paul S. Trible Jr. (R-Va.) next year; make himself available as a ticket-balancing vice presidential running mate; or pick up Nunn's mantle of strong defense views and fiscal conservatism to join the already crowded field of Democratic presidential hopefuls. So far, the popular former governor has steadfastly ruled out running for president.

"Robb is obviously a choice that many Democrats, including liberal Democrats, would like on the ticket because he has appeal in that part of the country where they have suffered some severe defeats," said Robert D. Holsworth, an associate political science professor at Virginia Commonwealth University here.

But, Holsworth added, Robb "seems to be portraying himself as a reluctant politician. His position is a curious one. He's going to have to make a decision about running for the Senate or other office long before the presidential nominee is selected."

Robb has championed one component of that presidential selection process, the "Super Tuesday" presidential primary in 14 southern and border states next March. He considers it a key test of the Democratic Party's commitment to issues affecting the South, and indicated that drumming up support for the regional balloting will occupy much of his time until then.

"Super Tuesday really becomes the pivotal date in all the things I've agreed to do," Robb said.

For Robb and other other southern Democrats who have staked so much on Super Tuesday, Nunn's decision to stay out of the nomination contest was particularly distressing. In the Georgia senator, the moderate-to-conservative wing of the national party had a politician for whom the primary appeared to be tailor-made -- in Robb's words, "an 800-pound gorilla preempting some of the turf."

Many party leaders also expect Jesse L. Jackson, a black southerner and likely candidate, to fare especially well in the March primary.

"Sen. Nunn would have made just the ideal candidate," said William F. Parkerson Jr. (D-Henrico), the Virginia Senate's most senior and quintessentially conservative member. "Nunn's quality and makeup were so positive that you were just ready to vote for him. You didn't have to have an argument with yourself."

Other Democrats regard a Super Tuesday without Nunn somewhat differently. "To view the southern primary as contigent on one candidacy or another is superficial," said Bernard V. Craighead, the southern regional political director for the Democratic National Committee and a former executive director of Virginia's Democratic Party.

"The important thing is that the southern Democrats who opted out of past presidential elections are now involved," Craighead said. "I hope Chuck Robb's still a player. His first choice {Nunn} is not running, but I would assume he would eventually find some other candidate."

Capps, the new president of the Southern Legislative Conference whose meeting Robb attended last week, went even further, saying, "I'm still hopeful myself that this {Nunn's decision} will make Robb do something. I hope he will reconsider and become a candidate."

Southern Democrats are looking for "a mainstream, middle-of-the-road candidate who they feel is not controlled by special interests, gay groups, unions," Capps added. "Robb is somebody we would be comfortable with."

Robb brushes off such talk, saying those entreaties are "sweet nothings that politicians hear whispered in their ear constantly."

"One individual told me after Sam's announcement, 'This is your time, this is the best shot you'll have,' " Robb said. "That may be so, but the more you know, the more you don't know. Everyone has their own sense of when they're ready to take on a venture."