Presidents tend to set the agenda for their parties. Most of the party's members of Congress tend to go along.
This has been increasingly the case as Americans over the last two decades have got out of the habit of splitting their tickets and have voted, in proportions not seen since the 1940s, entirely for candidates of one party or the other.
When Barack Obama first took the oath of office in January 2009, his fellow Democrats, with their large majorities in Congress, hurried to pass his key legislation.
The $787 billion stimulus package was passed in February. In June, the House of Representatives passed cap-and-trade legislation intended to reduce carbon emissions.
Obamacare took longer and was nearly derailed when Republican Scott Brown won the special Senate election in Massachusetts in January 2010. But in March, Speaker Nancy Pelosi and Obama rallied the House of Representatives to pass it anyway.
Democrats didn't need and didn't do much to seek Republican support. Three Republican senators voted for the stimulus. Eight House Republicans voted for cap-and-trade. Not a single Republican voted for Obamacare.
The large majority of Democrats voted for all three. Some paid a political price when Democrats lost 63 House seats in November 2010. But even after that, almost all Democrats continued to support Obama's positions on major issues.
Democratic voters went along, too. The 2012 exit poll shows that 92 percent of Democrats voted for Obama.
Now, suddenly, we are seeing some signs of Democratic discontent. The revelations of National Security Agency surveillance disturbed many Democratic voters and a not-inconsiderable number of Democratic senators and congressmen.
This was not the change they were seeking.
In the past two weeks, congressional Democrats have done more than express dismay. They have stymied two presidential initiatives on important public policies.
After Obama called for a congressional authorization of the use of military force in Syria, Democrats did not line up in large numbers in support. The whip counts of various news organizations and blogs showed some Democrats opposed and many Senate Democrats and most House Democrats as uncommitted.
The White House might have lined up enough to pass a resolution in the Senate. But with most House Republicans opposed, that seemed impossible in the lower chamber.
Obama's policy turnaround might of made this academic. Perhaps the unwillingness of Democrats to accept this agenda item may have undermined the credibility of any presidential threat to use force in Syria or elsewhere.