In an attempt to narrow the “insidious wage gap” between men and women, the California Legislature is on the verge of passing a stronger equal pay law that supporters believe will be the toughest in the nation — but some say still doesn’t go far enough.

Not only will the California Fair Pay Act make it more difficult for employers to justify paying men more than women for “substantially similar” work, but employers will be prohibited from retaliating against women who ask about or discuss the pay of their male colleagues.

“We’re closing all the loopholes,” said state Sen. Hannah-Beth Jackson, D-Santa Barbara, who authored the bill. “No more excuses.”

A growing debate and frustration over the gender pay gap has taken center stage in the Bay Area, and especially Silicon Valley, after recent legal battles have exposed how difficult it can be for women who challenge their male bosses for fair pay.

With support from Democrats and Republicans, as well as the California Chamber of Commerce, the bill that sailed through the state Senate is expected to pass handily through the Assembly on Thursday before landing on Gov. Jerry Brown’s desk for consideration.

The law would not require employers to share their salary data to help women compare their salaries to their male counterparts, however, even if women ask for it — a factor that women’s rights advocates say weakens the measure.

“Being able to get that data would be a more significant step than just being able to ask,” said Telle Whitney, CEO of the Palo Alto-based Anita Borg Institute that supports women in technology. “I think this is a baby step.”

The bill would, however, change the wording of existing law, that now requires equal pay for equal work, to equal pay for “substantially similar” work. While employers are still allowed to pay workers more money based on merit, seniority and other quantifiable factors, it narrows a loophole that now includes “any bona fide factor” as an excuse for paying a man more than a woman. That exemption was so broad that few women could successfully challenge it.

“In real life, it means that female housekeepers who clean rooms could legally challenge higher wages of those males cleaning the lobby,” Jackson said during a news conference Monday.

Pay equity laws have been on the books in California since 1949. John F. Kennedy in 1963 signed the Equal Pay Act that abolishes wage discrimination by gender. And this Wednesday marks the 95th anniversary of the day women were granted the right to vote.

But a wide wage gap persists, even in Silicon Valley which has long prided itself as a meritocracy. Women in technology make 70 percent of what men make, according to the Anita Borg Institute.

“What happens is that one by one, men go into the boss’s office and ask for a raise,” Whitney said. “Women don’t typically have the confidence, so they’re not asking for what they want. Women say, if I do a good job, I expect to be taken care of.”

But still, speaking out over perceived gender bias can backfire.

The legislation comes just five months after Ellen Pao lost her three-year battle with Silicon Valley’s most prestigious venture capital firm, Kleiner Perkins, in a case that made national headlines and highlighted Silicon Valley’s problems with gender equality.

Aileen Rizo, a single mother with a master’s degree, found the courage to file a complaint over pay inequity she says she suffered as a math consultant in Fresno.

“I know what it means to fight stereotypes, that you can earn higher education only to find out the man sitting next to you is taking home more money to his family,” said Rizo, who brought her three young daughters to the news conference. “What does society, the community, say about the worth of women when they refuse to pay them equal to their counterparts?”

Studies show that a woman in California working full time makes 84 cents to every dollar a man earns; that Latinas make just 44 cents for every dollar a white man makes; and African-American women are paid 64 cents for every dollar a man is paid, according to the San Francisco-based civil rights group Equal Rights Advocates.

What may be more startling is that even with equal pay laws on the books for decades, few women make claims against their employers. A review by Equal Rights Advocates found that in 2013, only six women in California filed claims over their pay.

“We have to ask why aren’t women saying anything about it?” said Jennifer Reisch, legal director at Equal Rights Advocates, which helped sponsor the bill. Part of the answer, she said, is that “there are very few women who actually know how much they make compared to men doing similar work.”

While the new law won’t force disclosure of men’s salaries, supporters of the measure hope it will begin to change the “culture of secrecy” in the private sector about salaries.

The Equal Pay Act attempts to root out not just intentional gender discrimination in the workplace, but subconscious discrimination “often without intention or evil motive on the part of the employer,” Reisch said.

Some common employment practices lead to pay inequities, including starting salaries which rely on the earnings of their last job as a basis.

“Over time, what we see is women consistently starting off behind men from the get go,” Reisch said. “What may start off as a barely noticeable difference over time really grows and is the reason why over a woman’s lifetime, a woman is losing close to a half a million dollars due to the wage gap.”

While the salaries of public servants are often made public, a “culture of secrecy” exists among the private sector, said Jackson.

“It is shocking,” state Rep. Kristin Olsen, R-Modesto, said, “that in 2015, we’re still having to have bills like this and press conferences like this to achieve pay equity in the workplace.”

Contact Julia Prodis Sulek at 408-278-3409. Follow her at twitter.com/juliasulek