Sen. Ted Cruz of Texas has landed on Roll Call's Top 10 Most Vulnerable Senators list, and had his re-election race moved into the “toss-up” column by the Cook Political Report this week.

The bad news for the failed 2016 presidential hopeful comes less than five weeks before Election Day, when his challenger, Rep. Beto O'Rourke, will attempt to become the first Democrat to win a Texas Senate race in 25 years.

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Roll Call listed Cruz among its most vulnerable senators seeking re-election for the first time this cycle. The list is otherwise largely composed of Democrats seeking re-election in red states. The outlet cited O'Rourke's large cash haul and improving poll numbers in a state that President Donald Trump carried by 9 points in 2016.

The Cook Political Report also moved Cruz's race into the “toss-up” column. The race had been rated a “lean Republican” race after starting out as a “likely Republican” win. Cruz was elected to the Senate in 2012 by a 16-point margin.

Trump White House budget chief Mick Mulvaney recently admitted that the GOP may be looking at a surprise loss in the Lone Star State and cited Cruz's problematic personality as a major liability.

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“There’s a very real possibility we will win a race for Senate in Florida and lose a race in Texas for Senate, OK?” he told GOP donors last month. “How likable is a candidate? That still counts.”

The polls still favor Cruz, albeit not by much. RealClearPolitics and FiveThirtyEight both have Cruz leading by about 4 points. Nate Silver's FiveThirtyEight site gives O'Rourke a 27.7 percent chance to win at the time of this writing. A Reuters/Ipsos poll released last month found the race in a virtual tie, with O'Rourke holding a slight lead.

“There’s a possibility it could happen. I’m not saying probable. But it’s possible,” University of Virginia's Center for Politics director Larry Sabato told Reuters of O'Rourke's chances for victory, based on their polling.

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Even if Cruz remains a slight favorite, his vulnerability in what was previously seen as a safe Republican seat has been compounded by O'Rourke's massive success at campaign fundraising.

The Dallas Morning News reports that Cruz raised $12 million over the summer, the best quarter of his re-election campaign so far. It's still unclear how much O'Rourke raised over the summer, but data shows he raised “far more” than $17 million. The Democrat has now raised more than $40 million in total, compared to $24.9 million for Cruz since early last year.

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"We've raised over $12 million, which is a record," Cruz told the Dallas Morning News on Thursday. "That's good news. That is a sign of conservatives all over the state of Texas are stepping forward and engaging in this fight. We're grateful for that. That bad news is I think it is very likely that Beto O'Rourke is going to report a quarter north of $30 million."

O'Rourke also appears to lead in the metric that Donald Trump touted as the most important during his 2016 campaign: crowd size.

<style="color:#ee2c1d">READ MORE: Beto O'Rourke & Martin O'Malley: the dark horse ticket that could beat Trump in 2020

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His campaign rally with country music legend Willie Nelson in Austin last month drew an estimated 55,000 people. According to organizers, it was the biggest crowd for a Democratic campaign rally since former President Barack Obama's campaigns. Even without Nelson in tow, O'Rourke has consistently packed event halls as he travels to all of Texas's 254 counties.

"So much is out of our control -- including the Kavanaugh hearings, whatever the president does, whatever happens locally in our communities," O'Rourke told the Dallas Morning News this week. "But I'm convinced that if we continue to do what we have been doing -- showing up everywhere ... I'm very optimistic. I feel very good."