Last April, more than 350 people travelled to Los Angeles, California to attend a conference against the Boycott, Divestment and Sanctions (BDS) movement. Organised by the pro-Israel group StandWithUs, attendees gathered to strategise how to combat BDS, which advocates for economic action against Israel to pressure the Israeli government over its human rights abuses.

One of the speakers at the event was Noah Pollak, head of the Emergency Committee for Israel. Pollak had a message for proponents of the BDS movement.

"While you were doing your campus antics, the grown-ups were in the state legislatures passing laws that make your cause improbable," he said.

Pollak's message referred to an increasingly effective strategy pursued by pro-Israel groups in the United States.

BDS advocates are facing a barrage of bills that condemn the movement as "anti-Semitic" and bar state governments from contracting or funding entities that support boycotting Israel.

The aim of the bills, say pro-Israel advocates, is to prevent state contracts from funding what they see as a discriminatory movement.

"Israel's diplomatic missions in the US have expressed full support and appreciation for legislative initiatives to boycott the boycotters," said Shimon Mercer-Wood, spokesperson for the Consulate General of Israel in New York.

"The BDS movement is racist, discriminatory and hostile to freedom of speech. It is ironic that some try to paint anti-BDS decisions as a limitation of freedom of speech, since the movement itself is dedicated to silencing Israeli voices in the public space," Mercer-Wood added.

Legislation that prohibits state funds from going to pro-BDS entities has been enacted in 10 states and is being debated in many more. However, in some states such as Virginia and Maryland, coalitions of free speech advocates and Palestine solidarity groups have banded together to defeat anti-boycott bills.

Freedom of speech?

The legislative measures are the most significant challenge the BDS movement has faced in the US.

Palestine solidarity activists say that the BDS movement is not anti-Semitic, and that the measures are an attack on free speech and seek to stigmatise action for Palestinian rights. There is also fear that the legislation could chill free speech by making people fearful of punishment if they support the boycott movement.

"The rash of anti-BDS legislation that we're seeing is really a frantic attempt to stifle the success of the BDS movement," said Josh Ruebner, the policy director of the US Campaign to End the Israeli Occupation, which has lobbied against such legislation.

The BDS movement started in 2005, when more than 170 Palestinian groups endorsed the call for boycott and encouraged people around the world to join their campaign.

The goal of the movement is to end the Israeli occupation of the West Bank, Gaza and Jerusalem, to ensure equality for Palestinians living in Israel and implement Palestinian refugees' right to return to communities they and their families were expelled from in 1948, when Israel was founded.

The call to boycott Israel has since been taken up by hundreds of progressive organisations in the US.

It has found success on college campuses, where Students for Justice in Palestine chapters have helped pass student government resolutions that endorse divestment from companies supplying the Israeli military.

It has also found success in academic associations and among churches, some of which have voted to divest pension funds held in companies such as Caterpillar and Hewlett-Packard, which sell equipment to the Israeli army.

In 2013, the American Studies Association endorsed the academic boycott of Israeli universities over their complicity in Israel's control of Palestinians. The move sparked a fierce backlash in the US.

The New York State Senate became the first body to pass a bill to prohibit state colleges from funding academic groups that boycott Israel.

The New York measure ultimately did not become law. But since then, a wave of anti-BDS legislation has swept across the country.

Twenty states have considered anti-BDS laws. Nine have enacted the legislation, and in June, the governor of one state - New York - issued a first-of-its-kind executive order against the movement, according to a count by Palestine Legal, a group that defends the right to advocate for Palestine.

The number of anti-BDS laws debated and passed in the past year is unprecedented in scale, said Rahul Saksena, a staff attorney at Palestine Legal.

"Organising that's happening on the ground is not just gaining momentum but winning the hearts and the minds of people in this country," said Saksena. "Israel advocacy organisations are seeing that, and it's alarming them."

While some of the bills are mere resolutions that put a state on record as being against BDS, others have more teeth to them.

States such as Illinois, New York, South Carolina and others have enacted measures that prohibit state pension funds or state contracts from going to companies or institutions that support BDS.

These measures require the state to compile (PDF) a public list - called a "blacklist" by pro-Palestine activists - of institutions that support the boycott. These entities could range from banks that have pulled investments out of the occupied West Bank to church groups that have voted to divest holdings in companies that contract with the Israeli army.

Attacking BDS

Civil liberties groups say that these measures are unconstitutional attacks on the right of activists to boycott Israel, and note that the US Supreme Court has upheld boycotts as protected advocacy under the First Amendment of the US Constitution.

But Eugene Kontorovich, a legal scholar who has helped to write some of the anti-boycott laws, told Al Jazeera: "The laws in question do not prevent or punish anyone for engaging in any speech. It is well established that states can require that companies that receive state money do not engage in what the state views as discriminatory activity, even when that activity is motivated by sincere beliefs."

The dispute over whether anti-BDS bills are constitutional will most likely end up in a US court.

READ MORE: BDS is a war Israel can't win

Meanwhile, the US Congress has also joined the attack against BDS.

Last year, Congress passed a trade law that includes language requiring the US to discourage European boycotts of Israel.

And in February 2016, Democrats and Republicans introduced legislation, backed by the influential American Israel Public Affairs Committee, that would authorise states to take funding away from companies that boycott Israel.

Ruebner, the policy head of the US Campaign to End the Israeli Occupation, expects that more anti-BDS bills will be introduced in the coming months, especially since 2016 is an election year.

Introducing pro-Israel legislation could curry favour with some voters and donors. But, he added, Palestine solidarity activists will continue to advocate and lobby against anti-BDS measures.

"Palestine solidarity activists, First Amendment rights activists, are fighting back against these bills, in many cases successfully," said Ruebner. "We're not going to allow our legislations to try to punish civil society for responding to a call for social justice."