In a dramatic shift, Gov. Charlie Crist signaled Wednesday he might veto a bill tying teacher pay to student test scores, even as Florida legislators prepared for a final vote on the politically polarizing measure Thursday.

Crist, who appeared to support the merit-pay plan as late as Monday, said teacher concerns are affecting his thinking. The plan would base teacher pay raises primarily on student learning gains on standardized tests, such as the FCAT, and eliminate tenure job protections for teachers hired after July.

The Republican governor, who is running for U.S. Senate, said he spoke over the weekend with a longtime friend who has a special-needs son. "And he's like, ‘How can my son have progress?' It's very challenging," Crist said, referring to the learning gain requirement. "So it's weighing on me heavily."

Crist was noncommital on whether he'd sign or veto the bill, which is a top priority for Republican legislative leaders. But the outpouring from teachers has clearly had an impact. "Shame on any public servant who doesn't listen to the people," said the governor.

Compare that to Crist's statement Monday, when he praised the House for holding an eight-hour hearing on the merit-pay plan and said he hoped to "have comprehensive reform bills on my desk for action this session."

The shift fueled Capitol buzz that Crist is weighing whether to run for Senate as an independent, rather than face Marco Rubio in a closed Republican primary. Crist trails in the polls and is increasingly estranged from the conservative GOP base.

Helping sway Crist on the tenure bill: teachers like Marie Angel Welsh, who teaches at Nova Middle School in Broward and was among dozens of teachers to testify against the tenure bill in Tallahassee this week. She told legislators she's taught at high-scoring and low-scoring schools, and teachers were "no less talented" at the latter.

Debate over the tenure bill reached the House floor Wednesday, the final legislative stop. Republicans and Democrats parried during a marathon question-and-answer session over the implications of using test scores to set teacher salaries. The arguments: Republicans say the bill would reward good teachers and weed out bad ones, while Democrats call it an unfair attack on teachers.

During Wednesday's floor session, Republicans voted down Democratic amendments to water down the provisions linking student scores to pay. The House vote comes today, following the Senate's 21-17 approval two weeks ago.

Andy Ford, president of the Florida Education Association, a teachers union fighting the proposal, said he's not sure whether the governor would sign or veto the bill. "He always gives himself room," Ford said. "He won't back himself into a corner. What I take away from this is, it's not over until it's over."

For Crist, a veto could anger conservatives but appeal to the state's moderate and independent voting blocs – his strongest base of support. Crist ducked a question Wednesday about whether he's considering an independent run for Senate, which would allow him to bypass the Republican primary. "I'm focused on the session," the governor said. "There will be time for other things later."

That's essentially the answer Crist gave last spring, when reporters peppered him with questions about a rumored U.S. Senate bid. He entered the race a week after session ended.

Political strategist Steve Schale, who ran President Obama's 2008 Florida campaign, said it would be difficult but not impossible for Crist to win as independent in a three-man race against Rubio and Democrat Kendrick Meek. "If any politician could pull it off, Crist is positioned to make a run," Schale wrote on his political blog, citing his "universal name ID and hefty bank account."