Mexico's Green Party is not very green. Yet like many "green" products on supermarket shelves, it has benefited from a very slick marketing campaign, one that has made it the country's fastest-growing — and arguably most controversial — party.

While it markets itself as a fresh solution to Mexicans' day-to-day problems, on all major issues the Greens vote in lockstep with President Enrique Pena Nieto's Institutional Revolutionary Party. But despite scandals over illegal campaign advertisements and giveaways, the tiny party is gaining strength in polls ahead of the June 7 midterm elections.

To intellectuals and activists calling for its dissolution, the Green Party is nothing more than a front for the PRI, as the ruling party is called, and further proof the president's party is abiding by an old playbook.

The PRI ruled Mexico uninterrupted for 71 years before losing the presidency in 2000. When voters soured due to economic crises, the party often used alliances with small parties to freshen its image and ensure its grip on power. This election cycle, the PRI seeks to hold onto a coalition that puts it just a few votes shy of a majority in Congress, despite its own corruption scandals and public outrage over issues such as the disappearance of 43 teachers college students last fall.

The midterm elections, in which 500 members of congress, 17 state legislatures, nine governors and more than 300 mayors will be chosen, are mostly viewed as a referendum on the president's performance half-way through his six-year term.

As the PRI loses ground in the polls, the Green Party has surged. And critics say that is no accident.

"We distrust and will always distrust the PRI because they try to cheat," said Gustavo Madero, head of the rival National Action Party. "Now, they're trying to use the Green Party to deceive people."

The alignment of the PRI and the Green Party is undeniable. The Greens are small, holding only about 7 percent of seats in Congress, but their block vote puts the PRI close to a simple majority. Most of the time, the Greens' voting record is indistinguishable from PRI's, and it has unquestioningly helped pass PRI's free-market reforms, such as opening the national oil industry to private investment.

In return, PRI candidates often run in alliances with the Greens and support the small party's gambits, like a ban on circus animals, which are popular with voters. The PRI's support is key for the Greens to keep tens of millions in government campaign funding by keeping them above a minimum vote level.

The party claims credit for pushing through environmental laws that have raised penalties on polluters, protected mangroves and inserted the right to a healthy environment into the constitution.

But most environmental groups in Mexico and abroad say the measures are superficial and accuse the party of using the environment as a marketing tool. In December, Greenpeace Mexico called the Green Party "offensive" for claiming it had been working with the NGO on environmental protection.

The federation of European Green Parties stripped the Mexican party of recognition after it demanded in 2009 that kidnappers be put to death, a position counter to Green ideology. The Mexican party subsequently lowered its demand to life imprisonment, a measure since enacted in at least one state.

Ironically, the Greens were fined for violating an election law that requires all campaign flyers to be printed on recycled or recyclable material, and one Green Party city council candidate in Mexico City was filmed head-butting an official to stop him from evicting squatters from an ecological reserve area.

While weak on environmental cred, the Greens have become skilled at using brainstorming, opinion polls and focus groups to identify what voters want. It successfully proposed a law allowing people who couldn't get medicines they need from their local public hospital to take their prescriptions to any government facility that has the drugs.

Other election proposals are just as marketable: better English and computer instruction in public schools, and scholarships for Mexico's poorest and most isolated students.

"I think the Greens' success has been its discipline ... in not making lofty promises that can't be kept," said Carlos Puente, a Green Party senator. "Nobody can say the Green Party has lied, or hasn't made good on its promises."

Perhaps, but they have also had a hard time avoiding controversy.

In 2013, Green Party leader Gonzalez Martinez was jailed for drunk driving and was caught on tape in what sounded to many like a negotiation over a $2 million payout in return for allowing resort construction in ecologically sensitive areas in Cancun when his party held the mayorship of the resort city. He denies that the vague conversation was about a bribe.

Recent opinion polls suggest the Greens may be making some inroads. The party briefly surged to third place in one poll earlier this year, with about 11 percent saying they would vote for the Greens. Later polls show the party may be falling back to its historical average level of about 6 to 7 percent of the vote. That could still be enough to give the PRI a majority in an alliance after the June vote. That was precisely the strategy the PRI used with the Labor Party in the 1990s to fend off or defuse growing opposition.

But Puente, the Green senator, denies his party is a mere extension of the PRI. It "has its own life, its own essence," he said.