Morales was expected to run for the U.S. Senate seat being vacated by retiring Sen. Phil Gramm, R-Texas, but less than an hour before the deadline Thursday he filed as a candidate for the Democratic gubernatorial nomination.
Morales offered no immediate explanation for his surprising change in plans.
"He's sort of sneaking around, not talking even to insiders in the Democratic Party and now filing for an office that surprises everyone and disrupts the Democratic Party's plans in Texas," said Cal Jillson, a political science professor at Southern Methodist University in Dallas.
"This is a very strange move and denotes a certain instability in Morales' political conduct."
Morales did not immediately return a call for comment on his decision Thursday.
Tony Sanchez, a Laredo millionaire, had been expected to easily win the Democratic nomination for governor after his selection by party leaders who valued his new face, Hispanic name and deep pockets. Sanchez said he was surprised by Morales' decision and expected a spirited campaign.
State Democratic Chairwoman Molly Beth Malcolm also expressed surprise at Morales' filing for governor, but she said a vigorous primary battle would be good for the party.
"Democrats will have several competitive primary contests -- and that's a sign of a healthy party," she said. "Folks know it's a good year to be running as a Democrat in Texas."
With the rise of the Republican Party in Texas in recent years, Democrats are looking for major breakthrough this year. All statewide offices in Austin are held by Republicans, and the highest-ranking Democrat is House Speaker Pete Laney, D-Hale Center.
Jillson said Sanchez has been a "lackluster campaigner" so far and that may have played a role in Morales' decision to jump into the race and leave the crowded Senate field.
"I think it's still a very strange move after months of saying that he was gearing for a run for the United States Senate and people sort of planning the Democratic ticket with him slotted in as a serious candidate ... all that is kind of thrown up in the air," the professor said.
The winner of the Democratic nomination will face Republican Gov. Rick Perry, who inherited the office as lieutenant governor after George W. Bush was elected president. He has no primary opposition and a Democrat would have to run an "almost flawless race" to beat him, Jillson said.
Although Morales has more statewide recognition than Sanchez after serving as attorney general from 1991 to 1998, the Austin attorney is not off to a good start in the opinion of Jillson. With only weeks until the March primary, he calls the race even right now.
Other candidates in the race for the Democratic nomination for governor are Bill Lyon, a Waxahachie businessman, and John WorldPeace, a Houston lawyer.
Morales' exit from the Senate race has boosted the chances of former Dallas Mayor Ron Kirk of getting the Democratic nomination, Jillson said. Kirk, the city's first black mayor, and Rep. Ken Bentsen, D-Texas, the nephew of former Sen. Lloyd Bentsen, are the front-runners.
Current Attorney General John Cornyn is the leading candidate for the Republican Senate nomination. Others in the race are Lawrence Cranberg, an Austin physicist, Douglas Deffenbaugh, a San Antonio lawyer, Rusty Lang, a Dallas doctor, and Dudley Mooney, a Santo rancher.