WASHINGTON » The Senate is ready to launch an emotion-charged debate on new gun restrictions, four months after the carnage at a Connecticut elementary school spurred President Barack Obama and Congress to address firearms violence.

In an opening showdown today, a vote was planned on an attempt by conservatives to scuttle the Democratic bill before debate even started. There were no real doubts the conservatives would be defeated and lawmakers would turn to the legislation, which would expand background checks to more gun buyers, toughen penalties against illicit firearms sales and offer slightly more money for school security.

With that defeat imminent, conservatives were saying they would invoke a rule forcing the Senate to wait 30 hours before it could begin considering amendments.

"Let’s get on the bill," said Senate Majority Leader Harry Reid, D-Nev., as senators prepared for the vote. He said lawmakers had to try preventing criminals and the mentally ill from getting firearms, adding, "This bill won’t stop every madman determined to take innocent lives. I know that, we all know that."

Senate Minority Leader Mitch McConnell, R-Ky., was supporting the conservative effort, saying the legislation would restrict the constitutionally protected rights of relatives and friends to sell firearms to each other.

"This bill is a clear overreach that will predominantly punish and harass our neighbors, friends, and family," McConnell said.

The roll call was coming a day after two leading conservatives, Sens. Joe Manchin, D-W.Va., and Patrick Toomey, R-Pa., unveiled a less restrictive compromise on federal background checks, requiring them for gun shows and online transactions but exempting noncommercial, personal transactions.

That deal was expected to give gun control forces an initial burst of momentum as debate begins. But the National Rifle Association, along with many Republicans and some moderate Democrats, opposes fresh gun curbs as going too far, and the road to congressional approval of major restrictions remains rocky.

"Those two leaders stepping up is a very good way to start," said Sen. Mary Landrieu, D-La., who is seeking re-election next year and has stressed her support for both the right to bear arms and reducing gun bloodshed. "How it ends, I don’t know."

Toomey said today he believes supporters of the proposal that he and Manchin have advanced will be able to beat back any filibuster attempt. "Beyond that, I just don’t know yet," he said in a nationally broadcast interview hours before the critical vote.

"The problems that we have are not law-abiding gun owners like Joe and myself," Toomey said on "CBS This Morning."

But he conceded, "There’s no panacea here."

In December, a gunman killed 20 first-graders and six staffers at Sandy Hook Elementary School in Newtown, Conn. Relatives of some victims were at the Capitol pressing lawmakers to back gun restrictions, and were holding a vigil outside the building where they were reading the names of recent victims of gun violence.

Expanded background checks are the core of the Democratic gun control drive. Other top proposals — including bans on assault weapons and high-capacity ammunition magazines — will be offered as amendments during Senate debate but seem destined for defeat.

The compromise between Toomey and Manchin, both owners of guns and "A” ratings from the NRA, was likely to improve the prospects that the Senate might expand background checks by attracting broader support. But debate could last weeks and it was not known what amendments to the overall bill, either constricting or expanding gun rights, senators might approve.

Neither Toomey nor Manchin predicted the Senate would approve gun legislation and each said his vote on final passage would depend on what the measure looked like when debate ends. Manchin said he would vote against the overall legislation if his compromise with Toomey was defeated.

Reid, D-Nev., said the first amendment will be to add the Manchin-Toomey compromise to the legislation.

The senators’ agreement also has language increasing firearms rights. That includes easing some restrictions on transporting guns across state lines, protecting sellers from lawsuits if buyers passed a check but later used a firearm in a crime and letting gun dealers conduct business in states where they don’t live.

Underscoring the difficult path gun curbs face in the GOP-run House, Speaker John Boehner, R-Ohio, repeated his plan to wait for the Senate to produce something and pointedly noted that the background check agreement had yet to pass Senate muster.

"It’s one thing for two members to come to some agreement. It doesn’t substitute the will for the other 98 members," he told reporters.

Said Toomey: "Criminals and the dangerously mentally ill shouldn’t have guns. I don’t know anyone who disagrees with that premise." He said expanding the checks wasn’t gun control, "just common sense."

Background checks currently apply only to transactions handled by the country’s 55,000 licensed gun dealers. Advocates of expanding the system say too many sales — the exact proportion is unknown — escape the checks, which are supposed to keep weapons from going to criminals, the seriously mentally ill, and others.

In a written statement, Obama said, "This is not my bill," adding that he wished the agreement was stronger. Still, he praised it as significant progress, saying, "We don’t have to agree on everything to know that we’ve got to do something to stem the tide of gun violence."

Gun control groups gave the deal warm but not effusive praise, noting that unknown details and some pro-gun provisions gave them pause.

The NRA said it opposed the agreement.

And in a letter to senators, NRA lobbyist Chris W. Cox warned that the organization would include lawmakers’ votes on the Manchin-Toomey deal and other amendments it opposes in the candidate ratings it sends to its members and supporters.