President Clinton is nominating Jeffrey Davidow, an assistant secretary of state and career diplomat, for the long-vacant post of ambassador to Mexico.

Davidow, a native of Boston, directs the State Department's Latin America bureau. He is a former ambassador to Venezuela and Zambia and also held diplomatic posts in Guatemala, Chile, South Africa and Zimbabwe. He has spent 29 years in the foreign service.The Mexico City post has been vacant since last June's departure of former Rep. James Jones, D-Okla. Opposition by Senate Foreign Relations Committee Chairman Jesse Helms, R-N.C., blocked the confirmation of Clinton's first choice to replace Jones, former Massachusetts Gov. William Weld, a Republican.

Helms supported Davidow when he was nominated for his current State Department job in 1996.