Mr. Ryan’s speech was to provide the biggest opportunity yet for the 42-year-old conservative to make the case for his vision — embraced by Congressional Republicans and much of the party’s intelligentsia — of how to rein in soaring entitlement spending, shrink the size of government and alter federal spending priorities. His admirers were expecting him to shine.

“Tonight, the American people — millions who may not know a lot about Paul Ryan other than the headlines that they’ve read — are going to get to know Paul Ryan the way many of us know him; as a serious policy thinker,” Senator Marco Rubio, Republican of Florida, said on ABC’s “Good Morning America.” He called Mr. Ryan someone who “brings a unique life experience and a unique perspective on some of the issues that we face because of his age, because of our generation but, at the same time, someone who is as deep and serious a thinker about our issues as ever before.”

Democrats were just as eager as Republicans to focus public attention on Mr. Ryan and his plans. A group of top Democrats arrived in Tampa to slice and dice Mr. Ryan’s budget. The Democratic National Committee took out a full-page ad in The Tampa Tribune to accuse Mr. Ryan of trying to “end Medicare as we know it” through his proposal to provide federal health insurance subsidies for future retirees so they can buy private insurance rather than enroll in Medicare. The party also has two billboards with similar themes in the vicinity of the convention hall and plans to have a plane pull a banner Wednesday night with the message: “Romney-Ryan: Wrong for the Middle Class.”

Even as the political sniping went on, the convention managers and the Romney campaign were closely monitoring events unfolding along the Gulf Coast just a few hundred miles away as Hurricane Isaac pummeled Louisiana with rain and wind, causing flooding and providing a stark televised contrast to the convention festivities.

Besides Mr. Ryan and Mr. McCain, the speakers were to include Senators Mitch McConnell of Kentucky, John Thune of South Dakota and Rob Portman of Ohio; Mr. Portman was among the top contenders for the vice-presidential slot that went to Mr. Ryan.

Mr. Ryan’s speech is unlikely to focus too heavily on the wonkier details of his budget plan, which has twice passed the House. The Romney campaign has been distancing itself from the proposal, seeking to make clear that the presidential nominee will be the one responsible for setting economic policy.