WASHINGTON — A front-runner under attack from all sides, Republican Donald Trump demanded that his party’s skeptical establishment embrace the inevitability of his presidential nomination as he stormed into Washington on Monday. Democrats responded by debuting a multi-pronged assault, shifting their rhetoric and resources against the man they expect to face in a contentious and ugly general election campaign. “If people want to be smart, they should embrace this movement,” Trump declared at a news conference, shrugging off passionate resistance to his candidacy from both parties. “I’m an outsider,” Trump said.