At least three of the Democrats’ signature gun bills are in danger of dying when they are debated on the Senate floor Friday.

It comes down to simple math: 20 – 3 = Fail.

Democrats have a 20-15 majority over Republicans and know they can’t afford to lose more than two members on any bill. The dynamic has led to an intensive lobbying effort.

“The math is not a secret to anyone, and we are a planning a historical, monumental effort to make a sound case against these proposals,” said Senate Minority Leader Bill Cadman, R-Colorado Springs.

The three gun bills Democrats have expressed the most concern about are measures that limit ammunition magazine rounds; ban concealed-carry weapons on campus; and assign liability for assault-style weapon damages to manufacturers and sellers.

Democrats Cheri Jahn of Wheat Ridge and Lois Tochtrop of Thornton, are opposed to all three. If another senator votes “no,” those bills are dead.

Several Democratic senators who were polled by The Denver Post said they are undecided. Others said Friday’s debate could change their minds on proposals that they are currently supporting or opposing.

“There’s only one bill I’ve made up my mind to support, and that’s universal background checks,” said Sen. Andy Kerr, D-Lakewood, who has a tough re-election in 2014. “I’ll be listening to the debate on all the others.”

Senate President John Morse, a Colorado Springs Democrat who is sponsoring the liability measure, said he believes all seven bills will pass, but, “We’ll wait and see.”

The nation is watching.

Colorado, a battleground state in the 2012 presidential election, is now a battleground state in the national debate over gun control. Witnesses for and against the gun bills under consideration have invoked two of the nation’s gun tragedies that happened in Colorado: the Columbine High School shooting in 1999 and the Aurora movie theater shooting in July.

Proof of just how high stakes the battle has become: Vice President Joe Biden called four lawmakers on the House floor on Feb. 15 during a debate on four gun measures, outraging Republicans who argue Democrats are getting their marching orders from the White House and New York Mayor Michael Bloomberg’s Mayors Against Illegal Guns.

The Colorado Senate on Friday will consider the four bills that have already passed the House, plus three Senate measures introduced last week. All seven bills passed two Senate committees this week on party-line votes with hundreds of witnesses hanging out in hallways and hearing rooms and opponents honking their horns for hours outside the Capitol.

Sen. Angela Giron, D-Pueblo, chaired one of those committees and voted for the three measures before it: universal background checks, charging gun customers for their background checks and banning concealed-carry weapons on campus. She said she is studying the other bills and is undecided.

She has been slammed on homemade billboards erected in Pueblo and had to bring in volunteers to sort through the more than 9,000 e-mails she received to figure out which ones came from her district.

“I’ve met with some really great people from Pueblo who have tried to help educate me on the issue,” Giron said. “I went out shooting for the first time in my life. I went to a gun show for the first time in our life.”

Jahn said she is more influenced by what happened after the 1993 “Summer of Violence” than she is by the thousands of e-mails and phone calls flooding her office. The legislature in a special session that year cracked down on juvenile offenders, making it easier to try to them as adults and lock them up for longer periods of time. Since then, various legislators, including Jahn, have worked to change those restrictions.

“There were huge, unintended consequences,” she said.

Of the current gun bills, Jahn said some are flawed.

“I don’t think you can enforce them,” she said.

Tochtrop said she supports only two gun bills, her proposal banning online training for concealed-carry holders and a measure charging gun customers for their background checks.

“I have always been a Second Amendment proponent. I carried the concealed-carry bill in 2002, although it didn’t pass,” she said. “I have looked at these bills very carefully, and we are not addressing the root of the problem, and that is mental health.”

Sen. Linda Newell, D-Littleton, represents a swing district with borders located 1 mile from Columbine and 1 mile from the movie theater. She said she is “struggling” with the liability bill and especially the measure banning guns on campus because of her background in helping battered and abused victims. That experience, she said, makes her empathic with women wishing to protect themselves.

“Our house was literally a backup for safe houses. My children grew up with people coming in and living with us for a few weeks to get back on their feet,” Newell said. “One the other hand, I’m getting feedback from people saying, ‘Wait a minute, these are college campuses. This is when there is a lot of alcohol, there is a lot of suicide potential, when mental illnesss starts to set in. We don’t want guns near us.’ “

Among those who were lobbying senators on the gun bills Wednesday was Christine Scanlan, a legislative lobbyist for Democratic Gov. John Hickenlooper.

The governor is on the record supporting three gun bills: universal background checks, requiring gun customers to pay for their checks and limiting magazine limits. His office said he is studying the other bills.

Among the lawmakers who aren’t subject to any lobbying is Sen. Irene Aguilar, D-Denver, who has made it clear she supports all seven gun measures. A medical doctor, she said most people in that profession support tighter gun restrictions.

Lynn Bartels: 303-954-5327, lbartels@ denverpost.com or twitter.com/lynn_bartels

Senate to debate seven gun bills

Senate Bill 197: Restricts firearms for domestic violence offenders

House Bill 1224: Limits ammunition magazines to 15 rounds

Senate Bill 196: Assesses liability for assault weapons

Senate Bill 195: Bans online training for concealed handgun permits

House Bill 1229: Requires universal background checks for gun sales and transfers

House Bill 1228: Requires gun customers to pay the costs of their background checks

House Bill 1226: Outlaws concealed-weapon permit holders from carrying on campus