From a group of 16 GOP senators gun control advocates have considered possible allies, at least nine have now said they oppose the background check compromise and one said he is leaning against it.
Combined with the 31 senators who voted against debating the overall gun bill last week, that could bring potential opponents of expanding background checks to 41 - just enough votes to block the Senate from considering the compromise. But in the heated political climate and heavy lobbying certain in the run-up to the vote, minds on both sides could change.
One of those expressing opposition was Sen. Jeff Flake, R-Ariz., who said Monday the measure would cover too many gun transactions.
"It would likely even extend to message boards, like the one in an office kitchen. This simply goes too far," he posted on his Facebook page.
Flake has been a primary target of pressure from gun control groups. He comes from the same state Giffords represented until she was severely wounded in a 2011 mass shooting in Tucson. In addition, Flake's senior colleague, Sen. John McCain, R-Ariz., said he is leaning strongly toward supporting the background check plan.
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 would still need eight additional votes.
So far, three Republicans who backed beginning debate have said they will vote for the Manchin-Toomey plan: Toomey himself and Sens. Mark Kirk of Illinois and Susan Collins of Maine.
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 both said they are still deciding on the Manchin-Toomey plan.