Massachusetts Gov. Deval Patrick plans to announce a spending proposal that retains medical coverage for some 30,000 legal immigrants who are at risk of losing it, and will also agree to ensure dental coverage for another 700,000 of the state's poorest residents. State-subsidized coverage for the two groups has been endangered as Massachusetts lawmakers struggled to craft a budget amid an economic downturn that has sharply curtailed tax revenues. The governor will propose the healthcare spending for legal immigrants as an amendment to the state's $27.4-billion budget.