WASHINGTON — A modest, bipartisan budget pact designed to keep Washington from lurching from fiscal crisis to fiscal crisis and to ease the harshest effects of automatic budget cuts is on the brink of passing the Senate Wednesday despite increasing political heat over a proposal to curb the growth of the pensions claimed by working age military retirees.The Senate is on track to clear the bill for President Barack Obama’s signature after a 67-33 vote Tuesday in which it easily hurdled a filibuster threshold.The measure would restore $45 billion, half the amount scheduled to be automatically cut from the 2014 operating budgets of the Pentagon and some domestic agencies, lifting them above $1 trillion. An additional $18 billion for 2015 would provide enough relief to essentially freeze spending at those levels for the year.The bill advanced with the help of 12 Republicans, several of whom promised to oppose the measure in Wednesday’s final vote because it fails to take on the nation’s most pressing fiscal challenges. It would barely dent deficits that are predicted to lessen in the short term but grow larger by the end of the decade and into the next.One provision, cutting the inflation increases of pensions for military retirees under the age of 62, was proving to be especially unpopular. Members of the military are eligible to retire after 20 years at half pay. The provision was included in the bill at the direction of House Budget Committee Chairman Paul Ryan, R-Wis.