WASHINGTON — Michelle Obama turned uncharacteristically political on Tuesday, pushing back against a measure pending in the Republican-controlled House that would let some schools opt out of federal dietary standards for school lunches.

The standards, approved by Congress and the president in 2010, set limits on sodium, fat and calories, and require that unhealthy menu items be replaced with fruits, vegetables and whole grains.

Some big food companies and Republican lawmakers have criticized the rules, calling them inflexible, ineffective and expensive. Representative Robert B. Aderholt, Republican of Alabama, has attached language to a spending bill that would waive the requirements for financially ailing school districts.

In response, Mrs. Obama met with a half-dozen school officials on Tuesday from New York, California, Virginia, Maryland, Georgia and Tennessee who attested to the success of the new standards in their school systems. The standards have been introduced gradually over the last two years.