A group of black parents have sued the Denver public school system, demanding the right to use state funds to send the children of low-income families to private schools of their choice.

The law suit has opened another battle front in the bitter US fight over school choice and voucher schemes. It highlights an unlikely alliance between frustrated minority groups in the inner cities and conservative interest groups pushing privatisation.

The Denver case demonstrates what both sides in the debate over vouchers say is an intensifying power struggle this year, reaching from the troubled school system in Washington DC, to other states from New York to Texas.

Meanwhile, the issue of whether government money can go to parents to pay private school fees, and particularly at church schools, is now threatening to make its way to the US Supreme Court. After an Ohio court ruled that a voucher scheme violated the constitutional ban on the separation of church and state, an appeal is likely.

In 1992 Colorado voters rejected a voucher initiative in the state by two to one. But the law suit involves only Denver, where black and Latino children between them account for two-thirds of about 73,000 pupils in public schools. That racial imbalance reflects a national pattern, where white parents have moved to the suburbs and private schools in the so-called “white flight”.

The law suit was brought by about 100 parents at two black private schools, one of them a church school. It was the idea of Joe Rogers, a former Republican Congressional candidate in Denver who is himself black, and has the backing of conservative lobby groups in both Colorado and Washington DC.

The suit alleges the school district is failing to provide a quality education and asks for a court order to correct the situation. It cites average scores for black and Hispanic pupils on tests of reading, writing and maths skills that are half those of white students, and blames poor discipline and schools’ inability to sack incompetent teachers.

Mr Rogers said: “Parents are completely and totally fed up with the dumbing down of standards. The bottom line is these parents believe their dollars should follow the child, not the other way around.” This week, he says, up to 700 more parents are expected to join the case.

In the past year, conservative reformers have been trumpeting the fact that black parents and some political leaders have come on board the school choice movement. Congressman JC Watts, for example, a black Republican from Oklahoma and a prominent figure in Republican circles since his election in 1992, has championed it. He cites polls showing that larger majorities of black and low-income people support school choice than among the white and better off.

Polls in Denver, it is said, back up these results. A large majority of Latino parents, and about half of black parents, support the notion of tax dollars going to the school of their choice.

Republicans are heavily promoting this link between conservatives and black parents, in what is surely an attempt to loosen the Democrat grip on the black vote.

The Heritage Foundation, a blue-chip conservative think-tank dating back to the Reagan years, recently devoted the cover story of its magazine to the subject, under the headline “Free at last. Black America signs up for school choice”.

In the city of Milwaukee, Harvard University researchers are reporting that students in a voucher programme are showing signs of substantial progress. The original Milwaukee voucher plan was introduced by Polly Williams, a local black politician who earned a college degree while raising four children. She was lionised by Republicans, including Congressman Newt Gingrich in the days before he became Republican speaker.

A black Democrat Congressman and former minister from New York, meanwhile, has been touted as a prominent supporter of school choice legislation on the national level. Floyd Flake broke ranks with the black leadership of his party to sponsor the DC Student Opportunity Scholarship Act of 1997.

It would offer scholarships worth $3,200 (Pounds 2,000) each to 1,800 students to attend the school of their choice. Backers insist it is not a school voucher programme, and would not take money from existing public schools, but it is surely a foot in the door. A similar Bill last year was so controversial that it was eventually blocked by old-time Democrat stalwart Senator Ted Kennedy in a filibuster.

Teachers’ unions have adamantly opposed voucher programmes in the US, saying they will undermine and break up the public school system, and create dual systems for rich and poor. The new head of the American Federation of Teachers, Sandy Feldman, recently used her inauguration speech to lash out against them.

In an interview this week, she accused conservatives, and particularly activists on the religious Right who want state funding to go to religious schools, of “exploiting legitimate demands and aspirations from parents”. The solution, she and other union officials say, is better discipline and higher standards within existing public schools.