Originally Published: 27 MAY 19 12:07 ET

By Brian Lowry, CNN

(CNN) -- Temporarily setting politics aside, "Running With Beto" is an intriguing fly-on-the-wall look at Beto O'Rourke's losing 2018 Texas Senate bid, and the nature of a grass-roots Democratic campaign in a red state. The HBO documentary functions less successfully as a window into the candidate, other than the sacrifices and occasionally frayed nerves that go hand in hand with such a pressure-cooker environment.

Like "Knock Down the House," the documentary featuring Alexandria Ocasio-Cortez's upstart run that recently premiered on Netflix, director David Modigliani captures the passion that powered O'Rourke's candidacy and his near-upset of Sen. Ted Cruz.

Filmed over the course of a year, that includes highs and lows -- the thrill of victory to the agony of defeat, from O'Rourke's gone-viral response about NFL players kneeling to teary-eyed campaign workers watching election results pour in, dashing their hopes.

The filmmaker enjoyed such extensive access to the candidate that the title can be taken literally, following O'Rourke as it does on his early-morning jog, as well as in quieter moments with his family. In the process, O'Rourke is shown occasionally snapping at staffers while lamenting time missed with his children as he crisscrosses the state in an effort to leave no stone -- and potential undecided voter -- unturned.

That portrait is almost relentlessly flattering, and as such, perhaps the least interesting aspect of "Running With Beto." There's more meat, rather, in those drawn to his message, from the volunteer clashing with her conservative family to another who posts road signs in what she calls "Trumpghanistan," eventually adopting a "Cry today, work tomorrow" attitude when it's all over.

The awkward part of "Running With Beto" is that it comes as O'Rourke has moved on to run for president. The result is that this film could easily be construed as a 90-minute infomercial, obscuring its broader insights about the nature of campaigning in the social-media age, and the possibilities of turning GOP-dominated Texas into a battleground state.

"We've gotta run like there's nothing to lose," O'Rourke says at one point, having stirred the imaginations of those who rallied around him with the soaring rhetoric that turned him into a national political figure.

Nevertheless, the documentary is most useful and effective as a snapshot of a moment in time, and the inherent challenges of campaigning as an underdog. "Running With Beto" works best, in other words, when viewed as a marathon, not a sprint.

"Running With Beto" premieres May 28 at 8 p.m. on HBO. Like CNN, HBO is a unit of WarnerMedia.