Wealthy donors poured millions of dollars into last-ditch efforts this week to derail Donald Trump's march to the Republican presidential nomination.

One of the leading anti-Trump groups, Our Principles PAC, more than doubled its fundraising to date just in the past week, said Tim Miller, a spokesman for the group. He wouldn’t give a figure, but federal records show Our Principles had spent at least $3.6 million prior to last week. The group is backed in part by Nebraska's billionaire Ricketts family.

Meanwhile, the conservative groups American Future Fund and Club for Growth reported a surge in contributions to back their own anti-Trump advertising, after Trump's victories in early primary states established him as the clear front-runner. Operatives working to stop Trump acknowledge that it's probably too late to prevent him from winning more primaries than his rivals, and their best hope may be to deny him a majority of delegates heading into the Republican National Convention in July.

“Just because it should have been done sooner doesn't mean you don't do it,” said Katie Packer, a former official in Mitt Romney's 2012 presidential campaign who is running Our Principles.

A key battleground is Florida, which goes to the polls on March 15 and will award all its Republican delegates to the top vote-getter. Most recent polls have Trump leading Florida Senator Marco Rubio by double digits there, but the operatives working to stop Trump contend the margin is narrower and that a wave of negative messages will eliminate it. American Future Fund is running a $1.75 million ad campaign in the Sunshine State this week, and Club for Growth is spending $1.5 million there. Our Principles hasn't said how much it plans to devote to the state.

“Everything hinges on March 15,” said Doug Sachtleben, a spokesman for Club for Growth. “If he were to have a solid day and win Ohio and Florida, it gets very difficult to press forward.”

For his part, Trump lashed out at Rubio during a Republican debate Thursday night, saying “the people of Florida can't stand him. He couldn’t get elected dogcatcher.”

Conservative Solutions, an independent group backing Rubio's presidential bid, is also pouring millions into advertising to attack Trump in upcoming primary states. Among its top donors are Paul Singer, the New York hedge-fund billionaire, and Larry Ellison, the founder of Oracle Corp. The group is slated to spend some $7.4 million between March 1 and March 15, including $4 million in Florida and $1.6 million in Illinois, according to Kantar Media's CMAG, which tracks political advertising. 

“We're running a multi-state, multi-million-dollar campaign against Donald Trump and the con-man scam he's pulling on the American people,” said Jeff Sadosky, a spokesman for the group.

Marlene Ricketts, part of the billionaire family that founded the TD Ameritrade brokerage and owns the Chicago Cubs, gave Packer's group its initial $3 million of funding in January. More recent donors won't be disclosed until later this month. Singer took part in a conference call this week to round up donors for Our Principles, as did Meg Whitman, the chief executive officer of Hewlett Packard Enterprise, according to a person with knowledge of the matter.

Richard Uihlein, a Wisconsin cardboard-box magnate, is another prominent donor who has disclosed support for anti-Trump efforts, giving $7,500 to Packer's group in January. He has also given $1 million to Club for Growth's super-PAC since it began running Trump ads last September, although it's unclear whether his donations were connected to them.

The latest round of anti-Trump ads argue that the billionaire real-estate developer would lose to Democrat Hillary Clinton in the general election, and that he's not the successful businessman he claims to be.

“There is a clear path to being able to stop Trump,” said Stuart Roy of American Future Fund. “It's possible. But the window's closing.”

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