(adds details on healthcare, Pentagon)

By Richard Cowan

WASHINGTON, Feb 25 (Reuters) - President Barack Obama sends his first budget proposal to the U.S. Congress on Thursday bracing for fights over how best to heal the economy, create a new healthcare system and still cut out-of-control deficits.

Obama can take some comfort in knowing that his fellow Democrats in Congress -- who control both chambers -- most likely will not pronounce his budget "dead-on-arrival," as has happened so many times to past presidents.

Even so, experts think that over the next several weeks, as lawmakers craft their own budget blueprint in response to Obama's request, the popular new president could be in for his first real fight with Congress.

"It won't be smooth sailing," predicted Maya MacGuineas, president of the Committee for a Responsible Federal Budget, which wants tough controls to bring down deficit spending and reform expensive programs like Social Security and the Medicare healthcare program for the elderly.

Obama, elected last November in part on a promise to foster fiscal responsibility, has seen Washington push through almost $1.5 trillion in emergency spending since October to fight a 14-month recession and a spreading global credit crisis.

He says the spending binge is crucial to head off an bigger economic disaster. But it has riled Republicans, who accuse Obama of ramming through Democratic priorities with scant attention to bipartisan support.

Obama will be giving an outline of his budget on Thursday and is expected to fill in the details in coming weeks.