Four senators may have signed on to a gun background check deal Wednesday, but only two showed up for the news conference — in part because Pennsylvania GOP Sen. Patrick J. Toomey’s public support for the bill hinged on not having to stand next to Democratic Sen. Charles E. Schumer of New York.

The background check bill, which would close the gun show loophole and expand checks to online sales, is officially co-sponsored by Schumer; Toomey; Joe Manchin III, D-W.Va.; and Mark S. Kirk, R-Ill. But only Toomey and Manchin appeared before the cameras. Toomey, the former Club for Growth president, had told Manchin he would not speak at the news conference if he had to get on stage with Schumer, according to two sources familiar with the talks. Schumer obliged, and Kirk also agreed not to appear in order to provide cover to Schumer.

“Manchin and Toomey are the best people to sell this agreement to the pro-gun community, and Schumer doesn’t want to get in the way of that. His job is to persuade the gun safety groups that it’s an acceptable deal,” a Democratic aide said of the group’s dynamic.

The aide added that Schumer on Wednesday morning called Vice President Joseph R. Biden Jr., New York Mayor Michael Bloomberg and Mark Kelly, a gun control advocate who is the husband of former Rep. Gabrielle Giffords, to encourage them to support the deal. And indeed, Giffords’ group, Americans for Responsible Solutions, did put out a statement Wednesday saying it was “pleased” with the agreement “that respects our 2nd Amendment rights.”

A spokesman for Toomey did not immediately respond for comment.

There was some apprehension from both sides that Schumer would become the face of the bill, which could effectively negate the conservative credentials Toomey brings to the table. And perhaps those fears were founded, because before the 11 a.m. Toomey-Manchin news conference, Heritage Action warned against supporting the background check measure, blasting an email “Senators Won’t Get Pass on Toomey-Schumer-Manchin.”

“Legislation drafted behind closed doors and rushed to the Senate floor has no place in our political system. We expect this type of deal making from Joe Manchin and also from Chuck Schumer, who supports the ‘universal registration’ of firearms. However, we expect more from Pat Toomey and, more importantly, so do his constituents,” the statement read.

The Wednesday news conference began on an awkward note, with Manchin acknowledging the “two Senators who aren’t here,” and then thanked his “friends” Schumer and Kirk. “Mark has been with me since the beginning and never left,” Manchin said. He then went on to describe the bill and his support of it, followed by Toomey, who did not mention Schumer.

Indeed, Toomey’s statement dealt less with the substance of the deal than it did with his reasoning that background checks do not constitute gun control.

“The common ground rests on a simple proposition, and that is that criminals and the dangerously mentally ill shouldn’t have guns,” Toomey said. “I don’t know anyone who disagrees with that premise.”

Meanwhile as Toomey and Manchin were speaking to reporters, Texas GOP Sen. Ted Cruz was criticizing their proposal on Laura Ingraham’s radio show, in another example of the political tightrope the GOP partners in the deal, particularly Toomey, must walk ahead of full Senate votes this week and next.

“Pat is a good man. I don’t agree with this particular proposal,” Cruz said, noting a disagreement over record-keeping of gun transactions. “In my view, maintaining a federal gun registry raises very serious constitutional concerns and is not consistent with the Second Amendment,” Cruz said.

Of course, the legislation as proposed by the bipartisan group does not establish a federal gun registry.

When asked by Ingraham if he was concerned about political consequences, he replied, “To be honest, I am not,” and then turned the issue back to Democrats. “I think a number of red-state Democrats who are up for re-election in 2014 are going to lose their seats” over this issue, he said.

Though he made the radio appearance, Cruz canceled his own scheduled morning news conference on guns.

Niels Lesniewski and John Gramlich contributed to this report.