WASHINGTON — (CNN) Spurred by the worst mass shooting in U.S. history, senators were voting Monday on new gun control measures that appeared unlikely to break the longstanding partisan gridlock that’s surrounded the issue for years.

Each party introduced amendments addressing similar concerns.

But none of the four measures was expected to reach the 60-vote threshold required for passage.

Two competing proposals — one from Democratic Sen. Dianne Feinstein of California and one from GOP Sen. John Cornyn of Texas — would set new hurdles for those on the federal terror watch list from buying guns.

But similar versions by those senators were rejected in December and neither is expected to pass.

In addition, senators will vote on one Democratic and one Republican proposal addressing background checks for gun sales, but members of both parties admit they are unlikely to be approved.

Ahead of the votes, both gun advocates and proponents of tighter restrictions sought to galvanize supporters.

“They’re blaming you, the (National Rifle Association) for the terrorist attack in Orlando and taking advantage of this tragedy to push their gun control agenda while emotions run high,” the NRA wrote to members Monday.

The group holds powerful sway among Republicans and moderate Democrats.

Senate Democratic Leader Harry Reid, meanwhile, blasted Republicans for withholding support for his party’s plans, saying on the Senate floor that “Senate Republicans should be embarrassed — but they are not, because the NRA is happy.”

Majority Leader Mitch McConnell touted the GOP proposals as more directly targeted toward the Orlando shooting, and argued the Democratic proposals would not provide due process for individuals barred from purchasing guns.

There were some Republicans who expressed support for Democrats’ plans, including Sen. Kelly Ayotte of New Hampshire, facing a tough re-election bid among moderate voters.

She said she’d back both the Republican and Democratic plans that would prevent people on terror watch lists from purchasing weapons.

That’s a position the vast majority of Americans favor — including 90% of Republicans, according to a CNN/ORC poll released on Monday.

That’s a higher figure than either Democrats (85%) and Independents (83%) who say they support such a rule.

Despite that support, however, neither the Republican nor Democratic plan appeared likely to pass amid the same rancorous atmosphere that has prevented substantial action on gun control following previous shooting massacres.

“Sadly the expectation is that you are not going to get enough Republican senators,” Florida Sen. Bill Nelson told CNN’s Alisyn Camerota on “New Day” in reference to the Democratic gun control proposals up for votes.

Before the first votes were cast, eyes were already turning to an effort by moderate Republican Sen. Susan Collins to craft a fifth alternative, one week after the Orlando terror attack. The Maine lawmaker was expected to unveil her proposal on Tuesday.

The Collins measure would bar people on the government’s no-fly list — a significantly smaller group than the terror watch list — from purchasing guns and also set in place some protections for anyone wrongly placed on the no-fly list.

Cornyn said Monday that McConnell is open to Collins getting a vote on her gun control proposal.

“I congratulate her for her good work. I just think these are the first votes were going to have, but that doesn’t preclude other votes. Sen. McConnell said if she wants a vote, I’m sure she could get one,” Cornyn — who introduced one of the four measures up for a vote Monday — said before voting began.

Collins’ proposal, however, has not garnered much support on the Democratic side.

Even if her gun control legislation does make its way out of the Senate, it’s unlikely to get a vote in the House, where House Speaker Paul Ryan threw cold water on the idea during his weekly press conference last Thursday.

Minnesota Sen. Amy Klobuchar told CNN’s Jim Sciutto that while there remains fierce resistance in the House — where Republicans outnumber Democrats 247-188 — the push for action in the wake of the Orlando terror attack reflects new momentum in the gun control debate.

“I don’t know that it’s going to be different than when we had votes in the past, but there is one thing that is different. People are starting to talk. There are starting to be negotiations going on. I think that’s very important,” Klobuchar said.