WASHINGTON — Rep. Anh “Joseph” Cao (R) of Louisiana must not have gotten the message from House Republicans that no one in the GOP caucus - repeat no one - would vote with Democrats on a sweeping overhaul of the US health-care system.

In a vote late Saturday night, Representative Cao - a vulnerable freshman in a Democratic district still devastated by hurricane Katrina - broke ranks, casting the lone Republican vote for the legislation.

“I have always said that I would put aside partisan wrangling to do the business of the people. My vote tonight was based on my priority of doing what is best for my constituents,” he said in a statement after the vote.

In Cao’s district, 3 out of 4 voters chose Barack Obama in the 2008 presidential elections. In 2004, President Bush won only 24 percent of the vote here.

The first Vietnamese-American elected to the US Congress, Cao last year defeated incumbent Rep. William Jefferson, after the eight-term Democrat was indicted for bribery.

For House Republicans, battered by the 2006 and 2008 election cycles, Cao’s victory last year was a rare bright spot. In a memo to colleagues entitled “The Time Is Cao,” GOP leader John Boehner of Ohio called Cao’s successful campaign, which targeted ethics, as the way out of the wilderness for Republicans.

Democrats pushed hard for Cao’s vote on health care, which gives at least a bit of bipartisan cover to the massive reform bill. Hours before the vote, Mr. Obama pledged to work with Cao on health-care issues in the district.

“Today, I obtained a commitment from President Obama that he and I will work together to address the critical health care issues of Louisiana including the FMAP crisis and community disaster loan forgiveness, as well as issues related to Charity and Methodist Hospitals. And, I call on my constituents to support me as I work with him on these issues,” he said in a message on his website. (The FMAP crisis refers to a sharp drop in federal matching funds for Medicaid, determined by Federal Medical Assistance Percentages.)

A former Jesuit missionary, Cao gave Democrats his vote also on condition that they include an amendment that explicitly bans federal funding for abortion services. Speaker Nancy Pelosi agreed to allow a floor vote on the amendment after tough negotiations Friday night. The amendment passed, 240 to 197.

Cao also said on his website: “Thanks to the Stupak-Pitts Amendment, taxpayer dollars will not go to supporting elective abortions, and for thousands of my constituents, this was a top priority. By incorporating this amendment into the health reform bill, my colleagues and I made this bill better, and that is an achievement of which I will always be proud.”