French leaders across the political spectrum today supported a declaration by Prime Minister Laurent Fabius refusing to give in to "blackmail" in the kidnaping of eight French citizens by extremist Islamic groups in Lebanon.

The expressions of political support for the government's firm line with the kidnapers came as France prepares to go to the polls Sunday for crucial parliamentary elections.

Many French commentators have already concluded that the Islamic Jihad organization and other militant Shiite Moslem kidnapers are seeking to humiliate Socialist President Francois Mitterrand in the same way that then-president Jimmy Carter was tested by the captivity of U.S. hostages in Iran.

Speculating that pro-Iranian extremists in Lebanon were trying to exploit the election campaign to increase the pressure on France, the Paris daily Le Monde said in an editorial that the government was preparing for "probably its most difficult week" since the left-wing election victory in May 1981.

Iran today refused to issue a visa to a French government emissary appointed to explore ways of securing the release of a four-man television crew kidnaped over the weekend in addition to four hostages seized last year.

Another French emissary, Dr. Reza Raad, has flown to Syria and is expected to go to Beirut Tuesday. Raad, a Lebanese-born French citizen who is an independent right-wing candidate in the current election, succeeded in making contact with the kidnapers of the four French citizens last year and has been named by Islamic Jihad as their only acceptable negotiator.

Neo-Gaullist leader Jacques Chirac said today that he supported the French government's declaration that it would not give in to the "blackmail" of the kidnapers.

Callers claiming to represent the kidnapers have demanded the release of five Arab prisoners held here for the attempted killing of Iranian exiles and the extrication of two Arabs whom France expelled to Iraq despite backgrounds as opponents of the Iraqi regime.

Chirac, considered a possible prime minister if the right wins in the elections, said it would be irresponsible to make an election issue out of "dramas that are rooted in the irrational, the fanatical and the violent."