WASHINGTON — Presidential contender Beto O’Rourke is helping a fellow Democrat raise money to unseat the Texas Rep. Briscoe Cain, after the Republican lawmaker tweeted last week that his AR-15 is “ready” for O’Rourke.

An email over the weekend from O’Rourke — still basking in the spotlight from his debate-stage vow that “Hell yes” he’ll confiscate assault-style weapons if elected — led to more than 3,600 donations for the campaign of Cain’s challenger, Josh Markle, of Deer Park.

Cain, a Republican from Deer Park, went viral after last week’s Democratic debate in Houston, when he tweeted at O’Rourke, “My AR is ready for you Robert Francis.” O’Rourke’s campaign reported the tweet to the FBI as a threat, then turned to its followers to raise money for Markle.

“Last year, Briscoe Cain ran completely unopposed,” the fundraising note, which vowed to split all money raised with Markle’s campaign, said. “This time, the Texas House Democratic caucus is running a campaign with Josh Markle to defeat him. The more money we can raise for them today, the stronger and clearer our message to Cain becomes. So please, make your best donation right now.”

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O’Rourke’s supporters apparently did just that.

Markle — whose first foray into politics was block-walking and making calls for O’Rourke’s 2018 Senate run — told the Chronicle that his campaign received more than $43,000 in donations from that email alone. He says his campaign has brought in nearly 4,800 donations, totaling more than $68,000, since Cain posted the viral tweet that put the race for the solid-red district on the map.

The attention has changed Markle’s mind about how realistic his chances are.

“I’ve been running and campaigning for six months and haven’t heard anything from anyone prior to this weekend,” Markle said. “I think with this energy we have now — from this weekend, from the tweet, and from Beto’s support — that this is definitely winnable now.”

Cain, meanwhile, has stood by his tweet — which Twitter removed because it violated the company's rules against threatening violence. “What I wrote really is just another way of saying ‘come and take it,'” he said on Fox News over the weekend.

Cain isn’t the only Republican who has expressed outrage at O’Rourke’s proposed mandatory buy-back.

“We now have three of the 10 Democratic presidential candidates actively advocating for gun confiscation,” U.S. Sen. Ted Cruz said on ABC’s “This Week” on Sunday. “They are saying that the federal government is going to come forcibly take your gun. So, look, I think that is a bad idea. The federal government should not be confiscating guns from law-abiding citizens.”

Despite the show of support over the weekend, Markle still faces an uphill battle. The last Democratic candidate who ran in the district, in 2014, drew just 22 percent of the vote. Cain drew more than 86 percent of the vote in 2016, his first race, against a Libertarian challenger. He ran unopposed in 2018.

As of the end of June, Cain reported $56,000 in his campaign account, fundraising records show.

ben.wermund@chron.com