Legislation that would permit the D.C. City Council to impose a D.C. income tax on commuters would set off a "regional war," Rep. Herbert E. Harris II (D-Va.) warned yesterday at the opening of two days of congressional hearings on the proposal.

Mayor Walter E. Washington, the leadoff witness, discarded his prepared text and denounced the "scare tactis" of Harris, with whom the mayor frequently has collaborated to confront metropolitian problems.

The sponsor of the bill, Rep. Ronald V. Dellums (D-Calif.) who presided as chairman of the House District Subcommittee on Fiscal Affairs, said he was "dead serious" about amending the home rule charter so that nonresidents could be taxed on income earned in the city.

The normally placid mayor spoke with fervor in support of Dellums' proposal, which is "more in line with our thrust for full home rule" than another bill, sponsored by Rep. Stewart D. McKinney (D. Conn), that calls for Congress to impose the tax on commuters.

"You can't say "you're free, now put your hands behind your back so we can tie them," Washington said.

Chiding Harris, the mayor said he wanted both hands free "to reach out and touch Virginia and Maryland."

We support a reciprocal agreement," the mayor asserted. "All you have to do is tax our folks who work over there. We want Virginia and Maryland to tax us. It's part of friendly relations, going down the metropolitian trail together."

Washington pointed out that Virginia and Maryland already have reciprocal taxing arrangements with 35 and 34 states, respectively.

Dellums' approach is viewed by some opponents as more serious threat than McKinney's because it focuses on the principle of home rule alone. McKinney's plan, approved by the subcommittee, has been attacked as another example of Congress interference in local affairs.

McKinney's bill would tax commuters at one-third the applicable D.C. rate. It would produce an estimated $60 million a year for the city. Under Dellums bill, the Council would be free to tax nonresidents at the full rate.

The issue may not result in metropolitian warfare, but it already has bought about alignment of natural opponents on the basis of geography.

Support for the mayor's position was echoed yesterday in testimony by his two main rivals for the job of mayor. Council Chariman Sterling Tucker and council finance Marion Barry.

Today, suburban and state officials, headed to put aside their partisan differences and testify against the bills.