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Ted Cruz is Rand Paul's biggest nightmare.
After his filibuster against drones this spring, Paul was the frontrunner for the Republican Presidential nomination in national polls, as well as polls in crucial early states like Iowa and New Hampshire. Now Cruz is taking support away from Paul and becoming the new leader of the Tea Party Republicans.
Yes, Cruz wants to attract Paul supporters. But Cruz's policy positions are very different and his strong style of leadership is completely different.
The stronger Cruz gets, the weaker Paul gets and the other way around.
Also, there is zero chance that Cruz would ask Paul to his running mate. Cruz will need to balance his ticket with someone from the Midwest, the West or the Northeast. As a Senator, Cruz will probably prefer to find a governor to run with for balance.