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Major Republican donors convened a conference call Tuesday afternoon, hours before polls closed in Super Tuesday states, to convince fellow donors to fund an effort to cripple frontrunner Donald Trump.

The call was hosted by Chicago Cubs co-owner Todd Ricketts, CEO of Hewlett Packard Enterprise Meg Whitman and hedge fund manager Paul Singer, who argued that it is not too late to stop Donald Trump. They warned, however: It has to happen fast, before the winner-take-all March 15 primaries cement Trump’s delegate lead.

Many former donors to Jeb Bush’s presidential campaign who don't have a candidate to back were on the call.

A key state is Florida, argued Brian Baker, a Ricketts associate who moderated the discussion, according to a call participant. Illinois and Missouri are other places where ads from anti-Trump super PAC Our Principles PAC (so far funded mostly by a member of the wealthy Ricketts family) could make a difference.

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Our Principles PAC had announced earlier on Tuesday a seven-figure ad buy to air nationally on CNN and on Fox in Florida, Illinois and Michigan attacking Trump over his now-defunct real estate school Trump University as being a "scam."

Donors have been reluctant to get on board with anti-Trump efforts out of skittishness around attacking him — and a failure to coalesce around a clear alternative candidate. Whitman backed Chris Christie; Ricketts had backed Scott Walker; and Singer is supporting Marco Rubio.

The argument to these donors now: Trump has only been stopped in places where anti-Trump advertising has been on the airwaves. Our Principles PAC advertised in Iowa, where Cruz won. The Club for Growth was up against Trump in Oklahoma, another state that Cruz won.

Stopping Trump requires limiting his victories in the winner-take-all states on March 15.

Baker, who manages much of the Ricketts’ political activity, walked donors through three weak areas that advertising could target, the source said:

His position on issues like immigration, where Trump has made contradictory statements.

His business record, like the failures at Trump University.

His controversial comments about race and in particular about women, which haven’t been part of paid TV advertising to date.

The question, however, is whether the urgency to stop Trump has escalated enough to convince donors to quickly pony up the millions of dollars that it would take an effort like this to be successful.

Skepticism among donors exists. One longtime Republican donor, Fred Malek, who was not on the call, told NBC News last week that efforts to try and stop Trump at this point won't work.

"If you did that I think there'd be deep-seated resentment at a group of wealthy donors telling people what to do," Malek said in a recent interview.

The leaders are following up with donors on Wednesday calling with inquiries or questions.

And some are enthusiastically on board. At least one asked: How can I wire the money so it arrives as fast as possible?