Gov. Tim Pawlenty quickly vetoed a bill Thursday that would have kept alive an expiring state health care plan for more than 30,000 low-income adults.

The program is ending April 1 because the Republican governor withdrew funding last year. Earlier Thursday, the Democratic-controlled House and Senate had overwhelmingly approved a bill to extend it.

Pawlenty vetoed the bill from Washington, where he’s on political business. In his veto message, the governor said it was “premature at best” to extend the program through June 2011, at a cost of $284 million, before the Legislature has addressed the state’s $1.2 billion budget deficit.

“Legislation that appropriates significant funds simply cannot be passed in a piecemeal fashion,” Pawlenty wrote. “A comprehensive, balanced budget solution must first be reached.”

The plan to continue a pared-back version of the General Assistance Medical Care program passed with bipartisan support in both chambers — 125-9 in the House and 47-16 in the Senate.

House Democrats who sponsored the bill said they would try to override Pawlenty’s veto next week. Democrats hold an 87-47 majority in the House, meaning they need at least three GOP votes to override.

“While we have a difficult budget to solve, I do not think this is the population that should pay the price,” said Rep. Erin Murphy of St. Paul.

Republican House Minority Leader Kurt Zellers predicted his members would uphold the veto. One of the GOP yes votes, Rep. Jim Abeler of Anoka, said he voted for the bill expecting it to go to a House-Senate negotiating committee where differences with Pawlenty could be worked out.

“When they go to do their override, they will discover there’s 47 Republicans who think the bill needs more work,” Abeler said.

General Assistance Medical Care covers adults with yearly incomes of less than $8,000, including many with chronic health problems, chemical addictions and mental illness.

Lawmakers aren’t satisfied with Pawlenty’s plan to transfer many of the affected patients into MinnesotaCare, another state health plan with more barriers to staying enrolled and less generous hospital coverage.

The temporary version of General Assistance Medical Care would have cut costs by paying hospitals, clinics and doctors less for taking care of patients. It would eliminate hospital coverage for higher-income patients and stop accepting sex offenders, illegal immigrants and pregnant women who qualify for other programs. The bill also would ramp up efforts to help veterans pursue other options.

Democrats said 85,000 adults would get coverage over 16 months, reflecting high turnover in a program with a monthly enrollment of about 39,000. They said their remake of the program would cost less per patient than Pawlenty’s plan to move them to MinnesotaCare, although the overall cost is roughly $30 million higher because theirs covers about twice as many people.