WASHINGTON — The Senate has voted to move ahead on a Republican budget plan — a critical step in the party’s drive to cut taxes and simplify the IRS code. The nonbinding budget plan would permit Republicans to pass follow-up tax cuts later this year that would add up to $1.5 trillion to the national debt over the coming decade. It calls for $5 trillion in spending cuts over that period, including cuts to Medicare, Medicaid, and the Obama-era health law, though Republicans have no plans to actually impose those cuts with follow-up legislation. The party-line tally sets up a vote later this week to pass the budget. GOP tax plans call for cutting tax rates for corporations, individuals, and business partnerships.