WASHINGTON — The Obama administration’s push for a smaller, nimbler military must now face the scrutiny of a Congress that has spent years battling the Pentagon’s vision for a new security strategy.

Defense Secretary Chuck Hagel is proposing to shrink the Army to its smallest size in three-quarters of a century, hoping to reshape the military after more than a decade of war in Iraq and Afghanistan and roped in by fiscal constraints set by Congress.

The plan unveiled Monday is already raising red flags among leading Republicans and Democrats.

“What we’re trying to do is solve our financial problems on the backs of our military, and that can’t be done,” said Rep. Howard “Buck” McKeon, R-Calif., the House Armed Services Committee chairman.

“There’s going to be a huge challenge,” Sen. Carl Levin, D-Mich., the Senate Armed Services Committee chairman, conceded.

Having backtracked just this month on cutting veterans benefits by less than 1 percent, lawmakers appear in little mood to weigh difficult, if necessary, decisions on defense reductions, especially as the nation gears up for midterm elections in November.

They have resisted cutting tanks and aircraft the military doesn’t even want, or accepting base closings that would be poison in their home districts. They have consistently advocated bigger pay increases for service members than the government has requested.

And although Congress has agreed on an overall number for the military budget in 2015, at just under $500 billion, there are still major decisions to be made on how that money should be spent.

“We are repositioning to focus on the strategic challenges and opportunities that will define our future: new technologies, new centers of power and a world that is growing more volatile, more unpredictable and in some instances more threatening to the United States,” Hagel said Monday at the Pentagon.

President Barack Obama will submit the budget to Congress next week.

At its core, the plan foresees the U.S. military as no longer sized to conduct large and protracted ground wars. Instead, more emphasis will be on versatile, agile forces that can project power over great distances, including in Asia.

The active-duty Army would shrink from 522,000 soldiers to between 440,000 and 450,000. That would make it the smallest since just before the U.S. entered World War II.

Other contentious elements include the elimination of the Air Force’s A-10 “Warthog” tank-killer aircraft and the Cold War-era U-2 spy plane; Army National Guard reductions; and domestic military base closings that Congress has roundly rejected since Obama became president. Military compensation will also decline slightly. Another flashpoint could emerge over the fleet of 11 aircraft carriers that the Pentagon insists it is maintaining.

“We are on a path to repeat the mistakes we’ve made during past attempts to cash in on expected peace dividends that never materialized” and “caused our allies to question America’s staying power and encouraged our enemies to test us,” Sen. Marco Rubio, R-Fla., said.