Washington ? No new president finds that every aspect of the job suits him at once; some duties are inevitably more comfortable than others. What we have witnessed in the last few weeks is Barack Obama trying on and fitting himself to the role of commander in chief.

The most controversial decisions of this period — expanding the troop commitment and replacing the commander in Afghanistan, opposing the release of photos of abused detainees, keeping the system of military tribunals, and delaying any change in the don’t ask, don’t tell policy on gays in the services — are of a pattern.

In every instance, Obama heeded the advice of his uniformed and civilian defense leaders and in each case but Afghanistan, he abandoned a position he had taken as the Democratic presidential candidate.

The predictable result has been the first sustained outcry from the left, angry denunciations from leaders of constituencies that had been early supporters. They feel betrayed as they watch him continuing, with minor modifications, the policies and practices of his Republican predecessor.

The political cost is not yet high, but those who remember Lyndon Johnson and Jimmy Carter know that over time, it can be dangerous for a Democratic president to lose the support of the liberal activists.

Whatever the risks, Obama clearly has taken on the mindset and the priorities of a commander in chief — and he is unlikely to revert. When Newsweek’s Jon Meacham asked him last week what was the hardest thing he’d had to do so far, Obama said: “Order 17,000 additional troops into Afghanistan. There is a sobriety that comes with a decision like that because you have to expect that some of those young men and women are going to be harmed in the theater of war.”

Some adaptation is necessary for almost every president because few experiences in their prior lives can really prepare them for the challenges Obama described to Meacham. George W. Bush went through it after 9/11, subordinating his domestic agenda to focus on the terrorist threat — and never changing.

But the step is harder for today’s Democratic presidents than for their predecessors — or their Republican contemporaries.

Ever since Vietnam — for more than 40 years now — the prevailing ideology of grass-roots Democratic activists has been hostile to American military actions and skeptical of the military itself. Iowa, where the Democratic nomination process begins, is famously tilted toward a pacifist view of war. Throughout the primaries, the pressures push forward candidates who do not challenge that mindset.

That was certainly the case last year, when Obama’s best credentialed challengers — Hillary Clinton, Joe Biden, Chris Dodd — all stumbled over their votes to authorize Bush’s use of force in Iraq.

The second reason Democrats struggle more with becoming commander in chief is that — unlike Republicans — they have more things they want to accomplish here at home. Time and money are always in short supply. The bigger the domestic agenda, the more resistance to being “diverted” into military adventures. Obama, like all his Democratic predecessors, has set out big goals. Afghanistan has to look like a distraction to him.

And a third reason is that today’s Democrats really are isolated from the military. Harry Truman had been an artillery captain; John Kennedy and Carter, Navy officers. But Bill Clinton did everything possible to avoid the draft and Obama, motivated as he was to public service, never gave a thought to volunteering for the military.

Nonetheless, circumstances made Obama the commander in chief of a nation fighting two wars. Consciously or not, he prepared himself for the transition by his choice of associates. He picked a vice president, Joe Biden, who had visited the battlefronts repeatedly as chairman of the Senate Foreign Relations Committee; a secretary of state, Hillary Clinton, who had immersed herself in defense issues as a member of the Senate Armed Services Committee; and a secretary of defense, Bob Gates, who had been running the wars for Bush. Then, most strikingly, as his national security adviser he chose not another of the academics who had customarily filled that role, but a very tough retired Marine general, James L. Jones.

They are the ones whose advice and counsel Obama has been heeding in recent weeks — not the political aides who guided him through the campaign and into the White House.

Obama’s liberal critics are right. He is a different man now. He has learned what it means to be commander in chief.

— David Broder is a columnist for Washington Post Writers Group.