WASHINGTON, May 14 — Despite weeks of steadfast rejections from Senator John McCain, some prominent Democrats are angling for him to run for vice president alongside Senator John Kerry of Massachusetts, creating a bipartisan ticket that they say would instantly transform the presidential race. The enthusiasm of Democrats for Mr. McCain, an Arizona Republican, is so high that even some who have been mentioned as possible Kerry running mates — including Senator Bill Nelson of Florida and Bob Kerrey, the former Nebraska senator — are spinning scenarios about a "unity government," effectively giving Mr. Kerry a green light to reach...