Alarmed by Donald J. Trump’s victories in seven states on Super Tuesday, Republicans desperate to sink his presidential bid moved on Wednesday to battle him on two fronts, attacking him with millions of dollars in television ads in Florida while girding for what would be the party’s first contested convention in 40 years.

The urgent new efforts, led by a group of corporate executives, financiers and Republican strategists, came as more party leaders expressed fear that Mr. Trump’s nomination would taint the party with a stigma of bigotry and recklessness. Yet many also acknowledged that it was now all but impossible to defeat Mr. Trump outright during the primary season.

Still, the focus on Florida, which holds its primary on March 15 — and where many of the anti-Trump forces also hope to see Senator Marco Rubio prevail, so he can continue in the presidential race — prompted fresh contributions to groups planning sustained attacks on Mr. Trump. In short order, three groups reserved $5 million of airtime in the state, a figure that is likely to grow.

In a preview aimed at sobering Republican voters, one of the groups, Our Principles PAC, released a web ad called “Unelectable,” presenting what it warned would be “coming this fall from the Democratic Party” if Mr. Trump was nominated: clips of journalists and entertainers seizing on a variety of Mr. Trump’s statements as racist, of the white supremacist David Duke expressing support for Mr. Trump, and of Mr. Trump refusing to condemn Mr. Duke.