Senate budget negotiators yesterday summarily rejected a House proposal designed to break a month-long deadlock over the $1 trillion fiscal 1988 budget.

The two chambers are divided over the level of defense spending. Senate Budget Committee Chairman Lawton Chiles (D-Fla.) called the House proposal to split the $13 billion difference between the House and Senate budget plans "a widening of the differences, not a narrowing."

The House plan, endorsed yesterday afternoon by the Democratic majority of the House budget panel, would have set defense spending authority at about $295 billion, $5 billion below an earlier compromise plan offered by congressional leaders and $6.4 billion above the figure adopted by the House in April. Liberal members of the House panel last week rejected the leadership compromise, contending it would lock in higher defense spending in future years and force domestic programs to bear the brunt of efforts to reduce the deficit.

The budget plan adopted by the Senate last month called for $302 billion in defense spending, $10 billion below the figure requested by the Reagan administration in January.

Chiles, after meeting with several senators on the conference committee, called the House offer a "nonstarter; it's not anything that any of our people would look at or consider."

Earlier yesterday, Senate Majority Leader Robert C. Byrd (D-W.Va.) said the Senate would not consider a military budget of less than $299 billion, saying to do so would jeopardize support from conservative Democrats whose votes are vital to final passage of any budget.

The $302 billion level was a crucial factor in getting the Democratic budget plan through the Senate last month on a narrow 56-to-42 vote.

"This is the bottom line," said Byrd, "because we are at the edge of a precipice where we may lose four or five {votes}."

House Budget Committee Chairman William H. Gray III (D-Pa.) argued that the House plan offered Senate conservatives a more reliable defense figure because the Senate budget resolution calls for shaving $7 billion off the $302 billion figure if President Reagan does not agree to an $18 billion tax increase. Reagan has vowed to veto any tax increases.

Gray said, "It appears that there is no need for further meetings. Each house will have to govern under different guidelines."

Under the House proposal, the fiscal 1988 deficit would be cut by about $41 billion, $4 billion more than any other spending plan yet considered.