What's behind Hart's sudden surge?

Gary Hart crisscrosses Florida and Georgia for three days this week as he scrambles for votes in the March 13 Super Tuesday showdown in nine states. As Senator Hart gathers speed in his race against Walter Mondale, some are asking: What is the secret of his surprising, sudden success?

Mr. Hart's associates say it's really not very complicated. The campaign is unfolding just the way the senator predicted it would last fall. His strategy has remained essentially unchanged.

The young Colorado senator is running a nonideological campaign. He shuns all labels - liberal, conservative, moderate.

Politically, that has great advantages. He's like a moving target. He goes left of Mr. Mondale, then he goes right. He woos voters from all parts of the political spectrum.

In doing so, he also avoids what his staff thinks was John Glenn's biggest mistake.

''We feel we're now at the point where Glenn was last fall'' when he was in a position to take over the race from Mondale, Hart aide Kathy Bushkin says.

Hart's advisers think that Senator Glenn made a strategic blunder in the October-November period. The Ohio senator had steadily gained on Mondale for months, and it looked as if he was in position to pull ahead.

Then Mr. Glenn began to nose-dive in the polls. What happened? Glenn, in an effort to pull ahead, began a series of attacks against Mondale. He was particularly critical of Mondale's ties with labor, of his record on military spending, and his approach to reducing the budget deficits.

Why was that a mistake? Because the way Glenn couched his criticism alienated the party's active liberal wing, and immediately began eroding Glenn's support. The liberal wing is extremely important in Democratic politics. In New Hampshire , for example, 44 percent of those who voted in the primary last week considered themselves liberals, according to exit polls.

Prior to his attacks on Mondale's liberal policies, Glenn was virtually as strong with Democratic liberals as he was with conservatives and moderates.

Hart doesn't plan to make that error.

Hart is ready and willing to blast Mondale. It's essential if he is to keep whittling away Mondale's support. But the criticism must be done in such a way that conservatives, moderates, and liberals all continue to give their votes to the senator from Colorado.

So far it's working. The proof is in the voter returns.

An analysis of the balloting in New Hampshire shows that Hart ran strong with all the major ideological groups. ABC-TV exit polls, for instance, indicated that Hart garnered 43 percent of the liberal vote. But he did almost as well (39 percent) with conservatives, and among moderates he captured 45 percent.

Hart was just about as strong with Roman Catholic voters (39 percent) as he was with Protestants (43 percent) and Jewish voters (42 percent). He was strong with people of English background (45 percent), Irish background (41 percent), and French (42 percent). He was almost as strong with union members (37 percent) as with nonunion workers (44 percent), even though Mondale had the backing of labor leaders.

To reach this point, Hart has proved himself to be a deft debater. He's won high marks from critics in forums in New York, New Hampshire, and Iowa, and remains the man to watch March 11, when the candidates next debate on TV from Atlanta.

His left-right strategy against Mondale could be seen in the last major debate, which was in New Hampshire. When Mondale voiced a few good words about the way President Carter handled the Iranian hostage crisis (''I'm proud of what we did''), Hart was was quick to retort.

The senator noted that the hostage rescue effort was a terrible failure; for this he blamed not being properly prepared. Hart's criticism hit home with conservatives (who want a more effective military) and liberals (who think much of the defense budget is poorly spent). It was an effective dig at the front-runner.