Rep. Beto O'Rourke, D-Texas Bill Clark | CQ Roll Call | Getty Images

Can Beto O'Rourke do what Donald Trump did to Ted Cruz? Probably not. But the improbable Democratic Senate candidate, just like Trump two years ago, aims to vanquish the hard-right Republican incumbent by surmounting the laws of ideology. O'Rourke's longshot bid in Texas reflects the broader challenge for Senate Democrats in 2018, when their hopes for recapturing the majority depend on winning a swath of conservative, Republican-leaning states. Yet the terrain for his battle, and the opponent he faces there, make it the most vivid test of the midterm campaign. Conservatism has reigned so completely in Texas that no Democrat has carried it for president in 42 years, none for Senate or governor in 28. Cruz made unyielding conservatism the foundation of his political career.

In 2012, he won the GOP Senate nomination, upsetting a far better-known Republican, by running to his right. After reaching the Senate, he begun seeking the 2016 GOP nomination by racing rightward aggressively and disruptively enough to make fellow Republicans loathe him. It seemed politically astute until Cruz ran into a wall he never planned for. Trump had once been a Democrat, a supporter of abortion rights, an advocate of tax increases — all positions believed disqualifying for hard-right GOP primary voters. Trump thrashed his conservative rival anyway with his celebrity, bluster, and visceral connection to working-class Republicans. By the end, Cruz bitterly labeled his conqueror an "utterly amoral" pathological liar. Now, in a different way, O'Rourke seeks to scramble Cruz's ideological formula again. Defying left-right conventions, the third-term Democratic House member stands with liberals on key issues such as "Medicare for all," restrictions on gun rights, legal status for immigrant "dreamers" and Trump's impeachment. His chances would be stronger, some political veterans maintain, had he courted Texans from the center. Yet he remains a threat to Cruz with his approach. His policy stances appeal to white liberals in big cities, millennials and Latino voters, a slumbering giant in state politics because their willingness to turn out has lagged behind their near-40 percent share of the population. Beyond that, O'Rourke has amassed extraordinary support for a Texas Democrat with his upbeat, high-energy style. O'Rourke shuns negative advertising and contributions from political action committees. As likable as Cruz is unlikable, he projects unrelenting positivity that tells Texans of all types politically "you're in the right place" at his rallies.