Senator John McCain intensified his criticism of President Bush and the administration's environmental polices on Tuesday, taking a walk in the cold, rain-drenched foothills of the Cascade Mountains and asserting that in the effort to stem climate change, ''America can lead and not obstruct.'' At an outdoor news conference in the Cedar River Watershed east of Seattle, Mr. McCain, the presumptive Republican presidential nominee, declared that ''the president and I have disagreed on this issue for many years -- it isn't a recent disagreement.'' He added, ''There is a longstanding, significant, deep, strong difference on this issue between myself and the administration.'' [ABSTRACT FROM PUBLISHER]