Is New York Mayor Rudy Giuliani eyeing higher office? The maverick Republican is doing lots of national travel and has plenty of campaign cash. Even GOP conservatives are singing his praises. One reason: Party strategists hope to entice Giuliani into challenging Senator Daniel P. Moynihan in 2000. Polls show that the popular mayor is the only Republican who could beat the veteran Democrat. But a source close to Giuliani says that Hizzoner doesn't want to be just one of 100 senators. He prefers an executive job, like governor (in 2002) or maybe even President (in 2004).