President Bush is set to put some muscle into his campaign rhetoric about how the global economic race is America's primary challenge for the 1990s. Washington insiders expect the White House to create a high-profile slot for an international-economic-issues czar if Bush is reelected. The leading candidate: Robert B. Zoellick, the former State Dept. economic policy whiz, who is now serving as Deputy Chief of Staff at the White House. The message: Economic competitiveness is a key pillar of national security.