Senate Democrats yesterday endorsed a plan under which the government would temporarily help home buyers make their mortgage interest payments. Their endorsement created a Senate majority in favor of the idea, proposed earlier by key Republicans as a way of shoring up the ailing housing industry.

The Reagan administration still is considering whether it should support the $1-billion-a-year mortgage assistance program, which sponsors are billing as a jobs stimulus and economic recovery measure.

The Democrats put the interest rate payment plan into a four-point emergency housing aid program. They also proposed government loans for down payments on new homes and loans to help homeowners keep from defaulting on mortgages, and urged that Congress reject some rescissions the president has proposed in past appropriations for housing for the poor.

Their objection to the rescissions is the only element in the Senate Democrats' plan affecting low-income housing. House Democrats, by contrast, have proposed an $18-billion housing program with much more emphasis on aid to low-income families.

The housing industry is in its deepest and longest depression since World War II, with unemployment in the construction sector at about 18 percent. Members of Congress increasingly are saying the nation cannot come out of recession without new housing aid that will let construction begin again.

"Among the many critical economic problems facing our country, none has more serious long-term implications than the crisis in housing," said Senate Minority Leader Robert C. Byrd (D-W.Va.). "The homeowners of America and the housing industry need a safety net to protect them from economic disaster."

Sen. Henry M. Jackson (D-Wash.), who headed a Democratic task force that developed the Senate Democrats' proposal, said it would stimulate construction of 600,000 homes, would create 867,000 new jobs and would keep 100,000 homeowners and families from defaulting on mortgages.

"The jobs represent mainstream America, . . . and mainstream America is in trouble," Jackson said at a news conference announcing the plan.

Under the Democrats' program and the Republican plan, introduced by Senate housing subcommittee Chairman Richard G. Lugar (R-Ind.), the federal government would pay to provide mortgages to moderate-income home buyers at 4 percentage points below the FHA interest rate, which is now 15 1/2 percent. The government's cost would be the difference between the subsidized rate the homeowners would pay and the market rate their lending institutions would receive. This could save a home buyer several hundred dollars a month in mortgage payments and so would enable many to buy a home that otherwise could not afford it.

The assistance eventually would be recovered from home buyers when the home was sold.

The Democrats' proposal differs from the Lugar legislation in targeting 30 percent of the aid to resales of existing homes and current new-home inventory. The Lugar bill would be for new home construction, not including inventory.

The other parts of the Senate Democrats' plan include:

Loans of up to $5,000 for down payments for first-time home buyers who can match the sum.

Loans for up to 21 mortgage payments for persons thrown out of jobs and facing foreclosure.

Rejection of budget rescissions on 50,000 previously authorized units of subsidized housing for low-income families.

The Democrats also proposed giving tax breaks to foreign pension funds to invest in mortgages here.