WASHINGTON — A bipartisan proposal to expand background checks to more gun buyers seems in jeopardy as more Republican senators are expressing opposition to the proposal, perhaps even enough to derail it. But there is plenty of time for both sides to change lawmakers’ minds.

As of Monday evening, senators were saying the vote appears likely late this week, rather than midweek as top Democrats hoped. A delay would give both sides more time to line up support.

Out of 16 GOP senators who voted last week to start debating gun control legislation, eight say they will oppose a background check compromise between a Democratic and a GOP senator. Two others say they are leaning against it.

Gun control advocates will need support from some of those GOP senators to prevail. Sixteen Republicans voted last week to reject an effort by conservatives that would have blocked the Senate from even considering a broad bill restricting firearms. With that debate now underway, Democrats hope to win enough supporters from this group to gain passage of the first amendment to that bill — the compromise between Manchin and Toomey — though more narrowly than Obama had hoped.

So far, seven Republican senators from that group have said they will oppose the Manchin-Toomey plan and one is leaning against it. Combined with the 31 senators who voted against debating the overall gun bill last week, that brings potential opponents of expanding background checks to 39 — just two fewer than opponents will need to sink the legislation.

“The Toomey-Manchin proposal, while well-intentioned, is not a solution to illegal gun violence. We already have major holes in the current” background check system, Sen. Lindsey Graham, R-S.C., said Monday in a written statement. Graham was among the 16 who voted to allow the debate to begin.

Opponents say expanded checks would violate the Constitution’s right to bear arms and would be ignored by criminals. They are forcing supporters of the background check plan to win 60 of the Senate’s 100 votes, a high hurdle.

Fifty Democrats and two Democratic-leaning senators voted last week to begin debate. If all of them support the Manchin-Toomey plan — which is not guaranteed — they will still need eight additional votes.

So far, three Republicans who backed beginning debate have said they will vote for the Manchin-Toomey plan: Toomey and Sens. Mark Kirk of Illinois and Susan Collins of Maine. A fourth, John McCain of Arizona, said he is strongly inclined to do so.

Sen. Frank Lautenberg, D-N.J., missed last week’s vote after saying he was suffering from muscle weakness, but spokesman Caley Gray said he hopes to be in the Senate for votes this week.

Two Democrats, both facing re-election next year in GOP-leaning states, voted against beginning the gun control debate last week: Sens. Mark Begich of Alaska and Mark Pryor of Arkansas.