Supporters of the Hanford cleanup initiative passed by voters in November filed a motion Wednesday asking to become a party to the legal challenge.

Initiative 297 prevents the Department of Energy from bringing more waste to Hanford until the waste already there is cleaned up.

But the U.S. government has questioned the initiative's validity, claiming it's written ambiguously, covering not just radioactive waste, but also any radioactive materials, including isotopes used for research.

The state is required to defend the initiative on behalf of the public. And attorney Michael Robinson-Dorn said his clients want to join in the state's case.

"Our involvement would provide the court with the perspective of those who drafted the initiative," said the law professor at the University of Washington.

Supporters include the Hanford watchdog group Heart of America Northwest; Sen. Adam Kline, D-Seattle; Rep. Toby Nixon, R-Kirkland; and Spokane City Councilman Bob Apple.

This week, state and federal attorneys asked federal Judge Alan McDonald to put the initiative on hold until its constitutionality can be argued in April. McDonald has yet to rule on the issue.

So for now, only shipments of most nuclear wastes to Hanford will be on hold. The rest of the initiative will not take effect until McDonald rules after the hearing or May 13, whichever is sooner.