As France prepares today for a decisive runoff round of parliamentary elections, a question rarely posed is what motivated some 4,000 candidates to run for the 491 seats of the French legislature.

French politicians, like their counterparts everywhere, are inspired by a variety of factors ranging from tradition and dedication to ambition and perhaps even the glamour of office, such as it is. And in France, as elsewhere, the climb to the top is tough.

During the period after World War II, many young men and women were catapulted into politics as a result of underground activities in the resistance against the German occupation of France. Nowadays, however, ascending the political ladder is a slower process.

The first step for most hopeful politicians is to get elected to the municipal council of their hometown. The next move is to become mayor, then member of the larger body that manages the department, France's basic administrative entity. Those positions put the rising politician on the path to national office.

Here, in contrast to the United States, a member of the national legislature is permitted to retain all his local jobs. He can even belong to the European parliament while serving simultaneously as members of the National Assembly, departmental councillor and mayor of his own town or city.

Under the Fourth Republic, which came to an end when Gen. Charles de Gaulle became President in 1959, a politician could also hold a ministerial portfolio in addition to his other functions. At present, cabinet posts go to technicians rather than to elected representatives.

This means, in effect, that the power of bureaucrats appointed by the president has increased sharply at the expense of the legislature. Thus the influence of the National Assembly is no longer what it was two or three decades ago, when prime ministers were replaced with revolving-door rapidity.

Almost all legislation is now drafted by the executive branch and introduced to the legislature by a prime minister beholden to President Valery Giscard d'Estaing. A member of the National Assembly may propose amendments, but the structure usually discourages such initiatives.

Legislators are futher held in check by their political parties, which demand total voting descipline. In theory at least, the majority party is the instrument of the president.

Members of the National Assembly can play a role in legislative committees. But committees are nowhere as important here as they are in Washington. They are generally understaffed, and like every other aspect of parliamentary life, they are dominated by political parties.

French legislators, who earn the equivalent of $43,000 per year, are well paid compared to standards here. Their working conditions, however, are primitive alongside the lavishness enjoyed by American congressmen.

Until a few years ago, members of the French legislature worked on their laps in the corridors of the National Assembly or held court in nearby cafes. Now, with the construction of a new building, they each have a one-room office, usually too small for visitors, who are received in the legislative bar.

A legislator also gets an allowance for an assistant and a secretary, and he can rely as well on help from his party's stenographic pool. He can travel free between Paris and his constituency.

A member of the National Assembly can pursue private activities, but within certain limits. If he is a lawyer, for example, he may not handle a case against the government. He is not allowed to hold a job in a nationalized company, nor can he serve as an executive in a corporation that has government contracts.

For the most part, however, legislators lack the time or energy to moonlight, and for that reason, political life is relatively unattractive to successful businessmen, lawyers and other professional figures, who would rather make money than serve in the parliament.

As a consequence, the National Assembly has become increasingly dominated by former civil servants, who are permitted to take a leave of absence from their jobs to run for office. If they lose their seat, they can return to the bureaucracy at their old rank and salary scale.

One discernible trend in all this is that many French legislators, frustrated by their impotence at the national level, are turning more and more to their localities, where at least they are still respected. They are honored guests at weddings, funerals and football games, which gain prominence through the presence of "Monsieur ie Depute."

In the process, though, the legislator appears to be assuming a new kind of responsibility. His constituents depend on him to obtain favors from the administration. So he has become a sort of lobbyist as he strives to persuade the bureaucracy to pay attention to the needs of his district.

But there is still enough prestige attached to membership in the National Assembly to make politics appealing. For those who have been in the legislature for years, politics is a virus that they cannot shake off.

Given France's present political instability, too, there is always the possibility that the system may eventually change and that the legislature, which had fallen into disrepute, may again emerge as the locus of authority. That hope drives many French politicians - and it suggests that, basically, they are not much different from their colleagues in other democracies around the world.