The proposed ban on the sale, transfer and manufacturing of many high-powered, semi-automatic guns in Colorado will face an uphill fight in the state Senate after clearing the House for the first time.

The measure, House Bill 1292, would ban guns referred to as “assault” weapons by its Democratic sponsors. It now heads to a Senate committee that includes one of the most vocal gun violence prevention advocates in the state — who happens to be a longtime skeptic of the effectiveness of blanket prohibitions like the one proposed.

Sen. Tom Sullivan is also the father of a victim of one of the state’s most high-profile mass shootings, and the committee’s tight partisan makeup looks likely to position him as the key vote.

The bill, titled “Prohibit Certain Weapons Used in Mass Shootings,” would define “assault weapons” as certain semi-automatic rifles and firearms, based on whether and where they take detachable magazines and have other characteristics, such as off-hand grips or collapsible stocks. It would not ban the possession of the weapons, meaning firearms covered under the bill but already owned would remain legal to keep.

The bill would also prohibit the possession of rapid-fire trigger activators.

In a historic first for the policy, the bill passed the full House on Sunday. On Thursday, it was assigned to the Senate’s State, Veterans and Military Affairs Committee, which will vote by early May on whether to send the measure to the full Senate.

Democrats hold a 3-2 advantage on the committee. Two of its Democratic members, Sens. James Coleman and Chris Hansen, both of Denver, expressed general support for the concept Thursday morning, though they added that they hadn’t yet fully evaluated the amended version from the House.

The third Democrat — and possible deciding vote, given universal Republican opposition — is Sullivan, who lives in Centennial.

He’s been a proponent of firearm regulation since his son, Alex, was murdered in the Aurora movie theater shooting in 2012, and he’s backed several gun regulation bills in the past year alone.

But he consistently has stopped short of supporting a ban on so-called assault weapons.

He worries the policy would spur a rush to buy the newly banned firearms, inadvertently increasing the number in circulation while not addressing the leading causes of gun violence. Other policies, he has argued, would do more to stem the tide.

Sullivan did not commit to a firm yes or no vote on Thursday. He reiterated his past skepticism and said he would be “crystal clear with everybody where I am” when the committee voted.

“If I had thought (an assault weapons ban) was the way to go, I would have done it the day after Alex was murdered with one,” Sullivan said.

“This is a gut-check issue”

Coleman, who chairs the committee, said he was leaning toward supporting the bill because “I don’t understand why folks need” the weapons. But he said he’s also keeping Sullivan’s perspective in mind.

“If the (desired) outcome is to save lives and keep people from getting access to stuff that can cause more harm … then we should be talking about modifications to firearms, and we should be talking about much more than just one particular type,” Coleman said.

Sen. Julie Gonzales, a Denver Democrat who’s the prime sponsor of the bill in the Senate, said she was in active conversation with Sullivan. But she wouldn’t say how she thought his skepticism could be overcome.

“It’s a hard thing to do, when he’s also seen, firsthand, the devastation that those weapons of war have on families and communities,” she said of Sullivan’s position as a potential make-or-break vote on the bill.

Gonzales said she was still working on gauging broader support among Senate Democrats.

“This is a gut-check issue for members of the Senate caucus,” she said. “… At the end of the day, no matter what happens on this bill, I think that our voters across the state are incredibly clear about their position on the issue, and I look forward to the next step of this process, which is the Senate committee hearing.”

Rep. Tim Hernández, a Denver Democrat and one of the bill’s prime sponsors in the House, echoed that sentiment. Sullivan’s long-standing skepticism of the policy is not new, and Hernández met with Sullivan before the bill was introduced earlier this session.

“I also know that as long as our colleagues follow through on (how) voters are saying they want us to handle issues,” Hernández said, “I have no doubts that we’ll arrive at the right place and land a bill on the governor’s desk.”

Governor also is “skeptical”

Gov. Jared Polis, also a Democrat, has also not embraced a ban on the high-powered guns.

He supported a federal ban during his time in Congress, and Polis spokeswoman Shelby Wieman said in a statement in recent days that federal action was needed to prevent gun violence.

Wieman wrote that though Polis is “skeptical” of the state ban, “he will continue to monitor it as it moves through the legislative process.” She did not directly respond when asked if Polis opposed the bill or would veto it, should it reach his desk.

Senate President Steve Fenberg, a Boulder Democrat, said he didn’t know the extent of support for the bill among Senate Democrats, though he was aware of “several no votes.” Should the bill pass its first Senate committee, it would need support from 18 of the chamber’s 23 Democrats to achieve a majority, given universal opposition from the chamber’s Republicans.

Fenberg indicated that he wouldn’t be willing to tweak a committee’s roster to ensure the bill made it to the Senate floor. Swapping members to ensure a bill’s survival or failure is uncommon, he said, and would happen only if a bill had enough support to fully pass the chamber.

“We just got the bill,” he said Tuesday, two days after it passed the House. “Honestly, I know the press and the House and Twitter has been talking about this bill ad nauseam since session started, but it hasn’t been much of a conversation in the Senate.

“I don’t think we have a great sense of exactly where people are. It just hasn’t been top of mind in the Senate like it has in the House.”

Sen. Mark Baisley, a Woodland Park Republican on the committee that will first hear the bill, said he would oppose the measure firmly both on the committee and on the Senate floor, should it make it that far.

“It’s obvious that the Democrat side of the aisle is far more in favor of gun control than the Republican side of the aisle,” Baisley said. But he said he was hopeful that “enough of the Democrats would join us in seeing reason, in adhering to their oath of office to uphold the constitutional rights that we all hold … and vote down that nasty bill.”

The bill is one of several firearm regulations being considered in the legislature this year, though it’s the only one targeting a specific type of firearm.

Other legislation includes bills banning firearms, even with a concealed-carry permit, at polling places, schools and government buildings, though local governments could opt out; requiring specific merchant codes for firearm and accessory sales; establishing state permits for gun dealers; setting insurance requirements for gun owners; and increasing training requirements to obtain concealed-carry permits.

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