Washington (CNN) -- Turn off the television, drink more water and serve smaller portions to fight childhood obesity, first lady Michelle Obama is urging as she launches a national campaign against childhood obesity.

Mayors, doctors and leaders in sports, entertainment and business will join the first lady as she kicks off the campaign Tuesday at a White House event. Members of President Obama's Cabinet will also be there.

The president is scheduled to sign a memorandum on childhood obesity in the Oval Office, the White House said.

During a speech last month at a Virginia YMCA, the first lady spoke about health issues caused by childhood obesity.

"We've seen the surge in obesity in this country is nothing short of a public health crisis, and it's threatening our children, it's threatening our families, and more importantly it's threatening the future of this nation," she said. "Higher rates of obesity are directly linked, as you've heard, to higher rates of chronic illnesses like heart disease and cancer and diabetes."

The number of overweight children ages 6 to 19 has tripled since 1970, a 2007 New England Journal of Medicine study found.

During her speech at the YMCA, Michelle Obama shared strategies she has used with her own children.

She helped her daughters stay active by limiting television watching, she said. The first lady also cut back on meal sizes and sugary drinks, and added more fruits, vegetables and water to their diet.

"It was really very minor stuff. But these small changes resulted in some really significant improvements," Obama said.