Democratic lawmakers urged President Barack Obama Monday to expand Social Security benefits for millions of seniors nationwide. In a letter to be delivered to the White House Monday, the lawmakers say shrinking employer retirement packages have made it more difficult for retirees to survive without additional Social Security dollars.

"As employers continue moving from a defined benefit model to a defined contribution model of retirement savings, it is critical that we fight to protect and expand Social Security -- the only guaranteed source of income in retirement," the lawmakers wrote. The letter came as the Obama administration announced Monday new programs and proposals at the White House Conference on Aging aimed at improving the quality of life and care for elderly Americans, including improving the health of family caregivers and free online courses for healthcare professionals about how to prevent patient falls.

"More than half (53 percent) of today's working Americans are not expected to have sufficient resources upon retirement to maintain their standard of living," the Democratic lawmakers wrote. An expansion of Social Security benefits would be enormously popular, they argued. "This support crosses party lines: 90 percent of Democrats, 73 percent of Independents, and 73 percent of Republicans favor expanding Social Security," the letter said.

The letter was signed by 2016 presidential candidate Sen. Bernie Sanders, D-Vt; Sens. Elizabeth Warren, D-Mass, and Sherrod Brown, D-Ohio; and 68 House Democrats. Liberal groups including the AFL-CIO, the Campaign for America's Future, the National Organization for Women and Social Security Works have also urged the Obama administration to allocate more funding for retirees.

Roughly 84 million Americans will be at least 65 years old by 2050. That's almost double the number of senior citizens living in the United States in 2012.

Most elderly Americans depend on Social Security benefits as a major source of income. The average monthly benefit is $1,294.