French President Valgery Giscard d'Estaing's reelection campaign got new momentum today with a full-fledged endorsement from Gaullist leader Jacques Chirac going far beyond the lukewarm support he gave immediately after first-round presidential voting 10 days ago.

Chirac's first statement was that, as part leader, he had no choice but to vote for Giscard but that his followers should let their own consciences guide them. A number of left-wing Gaullists than announced their support for Socialist candidate Francois Mitterrand.

Today, Chirac called on his electorate to reflect on "the risks that the election for France. . . . Mr. Mitterrand persists in taking inspiration from economic principles that have failed everywhere they were put into practice. Moreover, his alliance with the Communist Party bears risks that I have already denounced."

Chirac got nearly a fifth of the vote in the first round of the election, and Giscard needs most of those votes plus a portion of the relatively large stay-at-home vote to be elected. Detailed pools have shown that only about 70 percent of Chirac's voters were inclined to vote for Giscard, with the rest split between abstaining and voting for the Socialists.

In contrast, 85 percent of the Communist voters were shown to be ready to vote for Mitterrand in the runoff.

During a two-hour debate between the candidates last night, Mitterrand read several Chirac quotations critical of Giscard. Chirac's move today may help wipe out any benefit Mitterrand may have gained from the tactic.

As tempers from the first-round campaign have cooled, the Gaullist leader has come under increasingly heavy pressure both from the Giscard staff and from Gaullist parliamentary deputies to make a warmer endorsement of Giscard.

The Gaullist deputies have been abadoning their hostility to Giscard as it has sunk in, with many pointed reminders by Giscard campaign staffers, that Mitterrand has promised to call immediate parlialmentary elections. Giscard has pledged that the National Assembly would serve out its full term into 1983.