A new television commercial supporting President Reagan's proposal to arm the Nicaraguan rebels suggests that thousands of U.S. troops could be sent to "fight and die in Nicaragua" if the measure is defeated.

The 30-second commercial is the latest effort in a $1.2 million television advertising campaign launched last month by the National Endowment for the Preservation of Liberty in behalf of Reagan's $100 million aid package for the rebels, known as contras or counterrevolutionaries.

The commercial shows a young man, dressed in military camouflage fatigues, sitting at a desk writing a letter. "Dear Mom, it might be a while before you hear from me again," the letter says. "We're shipping out tomorrow and it's no secret to anybody where we're going. All I know about Nicaragua is what I read in the papers. But don't worry, Mom. I'll take good care of myself. And I'll make you and Dad proud."

Then an announcer is heard, saying, "If Congress will act while there's still time, thousands of Americans will never be sent to fight and die in Nicaragua. If we support President Reagan, this letter will never have to be written."

Spitz Channel, the president of the endowment, said the ad will be used in 25 markets in nine states and the District of Columbia. It is heavily targeted to the South, a major battleground.