Gilberto Hinojosa tells TMZ ... he truly believes the "Dallas Buyers Club" star and Oscar winner can beat Gov. Abbott in the next election, because along with polling well, he says Texans are eager for new leadership.

Hinojosa says Matthew can't go it alone though -- meaning he believes the actor won't win running as an Independent -- and says he's got no shot at winning the 2022 Republican primary because the Texas GOP is run by "right-wing extremists."

Gilberto tells us McConaughey is way too moderate for that crowd, but he could be the perfect fit for the Dems if he joins them soon ... and his political views line up.

For instance, Hinojosa tells us Matthew's stance on gun reform is strong and will help him gain support of Texans who are sickened by ongoing mass shootings ... 2 more of which occurred in the State this past week.

However, where McConaughey stands on fundamental issues for Dems like climate change, healthcare, education and abortion are less clear ... so Hinojosa's encouraging the actor to publicize those views and explain WHY he wants to be governor.

Then, if the Texas Democrats like what they hear, Gilberto says the party would do everything it can for him to make his campaign as simple and successful as possible.

Matthew needs to make up his mind relatively quickly though -- it's a huge state and will take a lot of time and effort to win ... and the deadline to throw his hat in the race for any party is December.

As we've reported ... the popular actor's openly mused about the possibility of running, saying "it would be up to the people." Then, some of those people -- 45 percent of a little more than 1,000 registered Texan voters -- said they'd vote for him if he runs for Gov.