Any tory leader, and I do mean any, would have lost by significant margins in 1997. The scale of that loss could have probably been improved with Major gone, but without some incredible political event, the tories were going to lose in 1997.
John Major wasn't a terrible campaigner or even politician, as evidenced by a respectable 1992 showing, but the mood of the country combined with 18 Tory years meant that a Labour win was inevitable. Maybe Portillo could have led the tories to a small win in 2001? But to be honest I am even unsure of that.