The result has been a subtle shift in which Democrats in Congress are becoming less deferential to the White House, making clear that Mr. Obama will not always be able to count on them to fall into line and highlighting how Mr. Obama's expansive ambitions are running up against political realities. The troop buildup is stirring unease among Democrats at a time when they have been struggling to navigate crosscurrents of pressure from different constituencies. Democrats now face the prospect of enacting a health care bill that Republicans are using to paint them as fiscally irresponsible and intent on extending the government' reach deeper into the economy and personal health decisions. Mr. Obama continues to push for action to curb global warming and to revamp immigration policy, subjects that seem certain to expose new geographic and ideological strains in the party, even as Democrats in Congress press the White House to make addressing high unemployment its singular focus. To a considerable extent, the strain stems from a calculation by Mr. Obama' aides that it is essential to move early in the term. But there are political calculations as well: Mr. Obama has nearly three years to recover from any damage he suffers by pushing through legislation that divides the public. Members of Congress do not.